Leviticus Leads to Confusion

So I am half way through Leviticus now and am totally confused on several different points.
1st point of confusion: Aaron. First he is a prophet but doesn't really do anything except what Moses tells him to do. I understand his part in the story at that point but then as soon as Moses goes up to Mount Sinai, Aaron is the one to build the golden calf! How can he go from being a leader for God to being the leading person in building the calf?! What confounds me even farther is that instead of killing Aaron like I expected God makes him a priest as well as all of his sons. This seems so different than the God that was shown throughout Exodus. In that Book, God was causing all sorts of pain to anyone who did not do as he said. Then later in Leviticus He kills Aaron's sons for using incense on the fire. That seems like a much smaller crime that leading all of the Isralites into worshipping a golden calf.

2nd point of confusion: What does "until the even" mean? It is repeated often through out the first half of Leviticus but I could not figure out what it meant for sure. Does it mean for ever?

3rd point of confusion: Why is childbirth considered to be unclean? After childbirth women were forced to go into isolation for a certain amount of time which depend on if they gave birth to a girl or boy. It just seems that it would be very weird that such a natural and necessary thing as childbirth be considered on the same level as having leprosy. Is it a continuation of the punishment of Eve? Is being considered unclean a sort of penance for Eve's sin? Personally if that is true, I would think that 9 months would be long enough of a punishment. If a woman gave birth to a girl she would be considered to be unclean for another 12 weeks?