The bronze serpent represented Christ on the cross. Bronze is associated with sin. Christ became sin and died in OUR place. Those who look to Him, are saved, those who don't, aren't. Pretty straight forward.
WHO is the Lamb of God? What was the lamb used for on the day of Atonement? How often did they have to do that? Once a year? So, I guess a literal lamb wasn't good enough, so they had to do it year after year. Christ was THE Lamb of God, He only had to do it once. Again, pretty simple and straight forward.
Septuagint. Not exactly sure what you mean by altered today. I would agree that many publishers have taken some liberties with the texts, by leaving out things or putting in things, but some try to translate the originals to more modern English.
They found some very ancient versions of Isaiah for example, that were basically intact, and, with very few minor exceptions, it is virtually word for word when compared to our modern day version.
Not my personal taste in music there dude, but whatever floats your boat I guess.