I will answer them my best on what i have read or have been told
1. A one piece base actually puts more pressure on the action, which in return effect accuracy. A gunsmith here (a very respectable one) did a test of how a 1 or 2 piece base effects the action when screwing on. I dont remember exactly how he explained it. but the 1 piece moved it around more than the 2 piece did. also a 2 piece base is lighter. and lighter makes it easier to carry.
2. you can get a 2 piece base with 20 moa, so no, going from a 2 piece base doesnt change anything of that sort. You can get 0 moa 15 moa 20 or 22 moa, and etc.
3. since you can get the 20 moa, there is no problem to deal with