ISAIAH - What is the justification behind the Doctrine and Covenants claiming that Jesus is the Meridian of time while the New Tesament claims that he was the fullness of time?

JOEL - You didn't give me a reference, but as far as I can see there are only two places in the New Testament where the words "fulness of time(s)" are mentioned, but mean two different things:

"But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law," (Gal. 4: 4)

In this case the phrase "fulness of the time" simply means that it was the right time for Christ to come to earth as recorded in the four Gospels. It is not naming the dispensation of His time.
We believe that there have been different dispensations throughout time that were presided over by either a prophet or Jesus Himself(Adam, Enoch, Noah, Abraham, Moses, Jesus Christ, Joseph Smith). By revelation we have learned that the name of Christ's dispensation is the "dispensation of the meridian of time" (D&C 20:26, D&C 39:3).
It is called "meridian" of time because we regard it as the middle or high point of that portion of eternity which is considered to be mortal time. The word "meridian" suggests the middle. According to Old Testament genealogies, from the Fall of Adam to the time of Jesus Christ was approximately 4,000 years. It has been nearly 2,000 years since Jesus' birth. The millennial reign will commence "in the beginning of the seventh thousand years" (D&C 77:12). After the Millennium there will be a "little season," the exact length of which is not revealed, but it could be several hundred years. In the context of these events, the Lord's mortal ministry took place near the meridian, or middle, of mortal time.
We believe we are now living in the "dispensation of the fulness of times" as talked about in the second reference where this is found:

"That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:" (Eph. 1: 10)

Paul is talking here about a future period of time that he designates as the "dispensation of the fulness of times", where all things are gathered together, including Christ's gospel and the gathering of Israel. We believe that our time now corresponds to this dispensation.

"It is necessary in the ushering in of the dispensation of the fulness of times, which dispensation is now beginning to usher in, that a whole and complete and perfect union, and welding together of dispensations, and keys, and powers, and glories should take place, and be revealed from the days of Adam even to the present time" (D&C 128:18).

ISAIAH - I have heard many times that Latter-day Saints do not believe that the crucifixtion played any role in the atonement and it only served use for the death of Jesus. Is this true? He did indeed suffer in Gethsemane yet many passages in the New Testament seems to indicate that the crucifixtion played an important role in the atonement as well.

JOEL - You heard wrong. We have never denied that the cricifixion was an important part of the atonement. If Christ had not suffered and died on the cross He would not have been resurrected and made eternal life possible for us; a very important part of the atonement.
We do believe that most of the suffering for our sins did occur in the garden. The Bible says while in Gethsemane:

"And there appeared an angel unto him from heaven, strengthening him.
And being in an agony he prayed more earnestly: and his sweat was as it were great drops of blood falling down to the ground." (Luke 22: 44)

Sounds like the Bible agrees that He did a lot of suffering in Gethsamane. Why else would He sweat blood and need an angel from heaven to give him strength? There were no helpful angels or sweating of blood mentioned at the cross.

ISAIAH - How come Joseph Smith claimed to have discovered an alter allegedly used by Adam, even though, over the years (approx. 6-7,000), vegetation would have accumulated over time and would have covered the whole area and it would have been impossible to have located it without having to dig extensively into the ground? The same for the New York hill (Mormon 6:6) where the golden plates were meant to have been deposited by Moroni in the 5th century A.D.

JOEL - Perhaps they were covered over by vegitation and dirt but then erosion exposed them again. Sorry I wasn't there to watch it happen.

ISAIAH - The book of Romans leaves no exemption to the sin and guilt that Adam passed on to all; no exceptions are made (Romans 5:12–15). Furthermore, it clearly states that "there is none righteous, no, not one" (Romans 3:10–12). How come then, in Moroni 8, the Book of Mormon claims that little children are incapable of (i) committing sin and (ii) being exempt from the curse of Adam?

JOEL - So are you telling me that my one year old granddaughter who does not yet know right from wrong, is capable of commiting sin? I haven't seen it happen yet.
Sin is the willfull violation of God's commandments. A small child might indeed unkowingly break a commandment of God, but you can't call it a sin until they are able to comprehend right from wrong and know what the commandments are and then purposely break them.
Here are the scriptures you refered to:

"Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
(For until the law sin was in the world: but sin is not imputed when there is no law.
Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam’s transgression, who is the figure of him that was to come.
But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many."(Romans 5:12–15)

"As it is written, There is none righteous, no, not one:
There is none that understandeth, there is none that seeketh after God.
They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one." (Romans 3:10–12)

I don't see how these scriptures say that Adam's sin and guilt were passed on to us. They only say that what Adam did brought death into the world and the capability for man to sin. But we did not inherit Adam's sin(transgression).
Notice it is called "Adam's transgression" and not "Adam's sin". A transgression is something a child might do, not knowing they are doing something wrong, which of course is the state Adam was in before he partook of the fruit.
The Apostle Paul is talking to adult members of the church here and is speaking in general terms about mankind.
However, the Moroni 8 scripture is talking specifically about little children.
Jesus said:

"Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven. (Matt. 19: 14)

A person with unrepented sins cannot go into Christ's Kingdom of heaven after death(Matt 4:17). Saying that little children are sinful is like we are forbiding them to come to Christ. What if they die before they know how to repent or are baptized?

ISAIAH - How come, in the Book of Mormon, (3 Nephi 12:2; Moroni 8:11) the remission of sins is the accomplishment of baptism: "Yea, blessed are they who shall be baptized, for they shall receive a remission of their sin. Behold baptism is unto repentance to the fulfilling the commandments unto the remission of sin," but in the (Doctrine and Covenants 20:37), the direct opposite is stated: "All those who humble themselves and truly manifest by their works that they have received of the Spirit of Christ unto the remission of their sins, shall be received by baptism into his church"?

JOEL - Baptism is performed for two reasons in our Church. The first is for the remission of sins as stated in the Book of Mormon; the second is for membership into Christ's Church as stated in the D&C scripture. So both scriptures are correct.
"Receiving the spirit of Christ unto remission of sins", doesn't mean the sins have been forgiven yet; it only means that the sinner has prepared himself by repenting of the sins and accepting the spirit of Christ so that baptism will wash away his sins.

"And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord." (Acts 22: 16)

ISAIAH - A "gift" is defined as "Something that is bestowed voluntarily, freely, and without payment" and "grace" is defined as "God's undeserved, unmerited favor." How come in D&C 14v6-7 it states "Keep my commandments in all things. And, if you keep my commandments and endure to the end you shall have eternal life, which gift is the greatest of all the gifts of God"? These seem to indicate that we must do something in order to have grace and the gift, notwithstanding their semantics indicating the opposite.

JOEL - Eternal life is a gift because we could never do enough to earn it on our own even though God wants us to do all we can so we can learn and grow from the experience.
Where did you get that deffinition of grace? How do you know it is correct? My deffinition of grace does not mean a free ticket into heaven. It means doing all I can to live a righteous life, and then having God make up the difference for me so I can obtain eternal life. I cannot do it all myself.

"For we labor diligently to write, to persuade our children, and also our brethren, to believe in Christ, and to be reconciled to God; for we know that it is by grace that we are saved, after all we can do. (2 Ne. 25: 23)

It's like the story where a father tells his child to work hard and save his money so he can buy a new bike. So the child works hard and earns and saves his money. The time comes to buy the bike but the child learns it costs $85.00 and he only has $10.00. The child is worried until his father says, "That's OK, because I love you so much, I will pay the difference for you".
The child gets the bike plus the growth and experience of working for the money he earned.
This is similar to how God's grace works for us. Please see my answer on salvation and grace at this page.

ISAIAH - Why was there an apostasy when the 3 Nephites and John held a valid priesthood and were meant to have been converting people (and are still meant to today) to the true gospel? Surely they would be (and have been) able to baptize people as they could convert them.

JOEL - I don't know. I'll ask them the next time I see them. Oh, and don't call me Shirley :-)

ISAIAH - In D&C 88, we learn that all light derives from Jesus in verse 7, etc. Isn't this scientifically innaccurate?

JOEL -
"Which truth shineth. This is the light of Christ. As also he is in the sun, and the light of the sun, and the power thereof by which it was made." (D&C 88:7)

Jesus created everything which means everything gets its power from Jesus, including the sun, the moon the stars, etc. Jesus, being God, can do whatever He wants whether it is scientifically accurate or not.
The "light of Christ" is not the same as the light from the sun in that scripture. The light of Christ is the truth spoken of in the sentence before that.
The light of Christ(also called the Spirit of Christ) is the spirit that eminates from Christ like light and gives all men the ability to know right from wrong.

"Wherefore, I beseech of you, brethren, that ye should search diligently in the light of Christ that ye may know good from evil; and if ye will lay hold upon every good thing, and condemn it not, ye certainly will be a child of Christ." (Moroni 7:18)
"For behold, the Spirit of Christ is given to every man, that he may know good from evil;" (Moroni 7:16)