JOEL - Here is the scripture in question:

"And the large and spacious building, which thy father saw, is vain imaginations and the pride of the children of men. And a great and a terrible gulf divideth them; yea, even the word of the justice of the Eternal God, and the Messiah(1830 ed. = Jesus Christ) who is the Lamb of God, of whom the Holy Ghost beareth record, from the beginning of the world until this time, and from this time henceforth and forever." (1 Ne 12:18)

It all depends on one's point of view in how we look at changes like these in the Book of Mormon(eg. Messiah vs Jesus). To a critic it can look like a mistake was made by a person who was just making the whole thing up.
On the other hand if you believe the Book of Mormon to be inspired scripture there could be a number of other logical reasons for the change. For example, in the case of 1 Ne 12:18, during translation, when Joseph Smith came to the character representing the Christ, instead of using the more correct translation of the Hebrew word "Messiah", he used the Greek name "Jesus Christ", because he knew that was the name of the Messiah and wanted to make it clear to the reader who the scripture was refering to.
Or perhaps after dictating the correct translation(Messiah) to his scribe, Oliver Cowdery, he remarked that the prophet was talking about Jesus Christ, so Oliver wrote down Jesus Christ instead of Messiah.
After the Book was first published he no doubt realized that at that point in time(1 Ne. 12) the name of the Messiah(Jesus Christ) had not yet been revealed as far as the Nephites were concerned, so he made the correction to Messiah to keep things in proper context and to better reflect the more proper translation of the Reformed Egyptian character.

King Benjamin said:
"And he shall be called Jesus Christ, the Son of God, the Father of heaven and earth, the Creator of all things from the beginning; and his mother shall be called Mary." (Mosiah 3:8)

Providing Christ's name does not sound like any new revelation King Benjamin is proclaiming to the people here. If you read this in context with everything else he said, he was simply reminding them what the name of the Messiah would be (The name "Jesus" is mentioned many times before this speech; 2 Ne 25:19-20, 26:12, 30:5 31:10, 33:4-6, Jacob 4:6), along with all the other new information that was revealed to him about His coming(read the whole chapter). And perhaps there were some listening who did not yet know it. Our latter-day prophets today often remind us of prophesies many times in their writings and talks they give in our conferences.

JAMES - In Alma 36 Alma claims he was unconscious for three days ands three nights. Why did the Mosiah version give two days and two nights?

JOEL - Here are the relevant scriptures:

"And it came to pass that I fell to the earth; and it was for the space of three days and three nights that I could not open my mouth, neither had I the use of my limbs." (Alma 36: 10)

"And it came to pass after they had fasted and prayed for the space of two days and two nights, the limbs of Alma received their strength, and he stood up and began to speak unto them, bidding them to be of good comfort:" (Mos 27:23)

Obviously the scripture in Mosiah only says that the people fasted and prayed for two days and two nights. It doesn't say how long Alma was paralyzed. He could have been in that condition for a whole day before they decided to start fasting and praying.

JAMES - Lions do not live in forests. Was 3rd Nephi 20:16 a mistake?

JOEL - Here is the scripture:

"Then shall ye, who are a remnant of the house of Jacob, go forth among them; and ye shall be in the midst of them who shall be many; and ye shall be among them as a lion among the beasts of the forest, and as a young lion among the flocks of sheep, who, if he goeth through both treadeth down and teareth in pieces, and none can deliver." (3 Nephi 20:16)

Jesus is simply reminding them of prophesies that were written long ago in the Bible by Isaiah and other prophets and probably also found in the brass plates brought over by Lehi.
In this case He is repeating the prophesy of Micah, who said:

"And the remnant of Jacob shall be among the Gentiles in the midst of many people as a lion among the beasts of the forest, as a young lion among the flocks of sheep: who, if he go through, both treadeth down, and teareth in pieces, and none can deliver." (Micah 5: 8)

So did the Bible make the same mistake? I don't think so. There are many kinds of cats that can be called "lions" that do live in forests. I once spotted something called a mountain lion in the forests of the Sierra mountains.