Wonderful questions, cj! And you even asked them on this thread (instead of the ‘fundie’ one)!
Why was another set of scriptures…needed?
We believe that God is willing to keep telling us things. The BoM, the Doctrine and Covenants, the Pearl of Great Price all represent scripture given to the world at various times to add to the Bible so that we can keep learning. The BoM people did not have the NT; they wrote down their experiences with Christ, and that has been passed to us so that we can know more of the story.
There is always more to learn. We believe that God will tell us more, and that more will be revealed. We think that there is other scripture given to other people in the world, perhaps lost, which we do not have yet (to answer your Korea/Australia question), and which we hope to have someday. The relevant Article of Faith reads: 9 We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God.
The Bible is great–we just don’t think that God has written the last word. We want all the scripture we can get.
As for why it took so long, we think (as you probably know) that the early Christians fell away from the true gospel after a little while, and lost priesthood authority. Luther and other reformers tried to bring it back, which we certainly honor them for. But the world just wasn’t ready for the Restoration until the 19th century–imagine what would have happened to a Joseph Smith in, say, 15th century France. Even in the US, where religious freedom was supposed to be guaranteed and oddball churches were springing up like weeds, the early Mormons were hounded and persecuted.
Please explain proxy baptism more fully.
First, let’s look at the famous 1 Cor. 15:29, which reads: 29 Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?
So we can see that Paul mentions it as an accepted practice in the early Church. And here’s why:
a) Not everyone who has ever lived on earth has had the opportunity to accept Christ as Savior. If this life is the only time we get to do that, then a lot of people are not saved, simply because they were never taught the gospel. This is not very fair, is it?
b) We believe that baptism is necessary for salvation (Mark 16: 16: He that believeth and is baptized shall be saved; but he that believeth not shall be damned), and that furthermore, baptism must be performed by the authority of the priesthood. Problem: many people have not been baptized, and even more have not been baptized with authority.
So. We have proxy baptism for the dead. We believe that so far, those who have died have not yet been judged for eternity (Judgement Day, at the end, has not yet occured); they are in the spirit world, where they either have the opportunity to learn more of the gospel, or to teach more of it. All of these people have free agency, of course, and may accept or reject what they have been taught. But they have no bodies–and baptism being a physical ordinance, it has to be done by a living person with a body. So we do it for them, and then they may choose whether or not to accept it and progress. Apparently, the early Christians felt similarly.
It may help you to understand that we don’t see what we’re doing as trying to get them to conform to Mormonism specifically (as you might try to get someone to be a Baptist vs. an Episcopalian here on earth). It’s a simpler question of the gospel of Christ, or not. Obviously you’re not going to agree with it, but that’s how we see it.
As a matter of fact, you don’t have to agree with me to be a good person. So there. 

