Different players have different opinions on how the different tactics work but these are my opinions:
1. Yeah they won't shoot 50% unless they are very lucky, and probably lower than 43% just because average isn't a good JS rating on its own regardless of defense. I know that was just an example, so in general if the defense equals the JS and the JR is decent I'm gonna say your assumption is correct, but I do not know this as a fact.
2. When I just started playing BB i was pretty concerned about where OD and ID draw a line, and I sort of forgot to find out for sure, but in my experience I would say that a 15 footer would be in the OD range. Layups would count as ID, and actually shot blocking would come into effect. Posting up would obviously be ID. So to have players that can defend in their primary defensive position and a decent amount in the other would be very beneficial especially in zones when there are switches all the time. When you say wouldnt a zone almost always be better for defending layups, I'm not exactly sure what you mean because there are outside oriented zones.
3. Yes if one of them is a better defender and you want him on his best offensive player for most of the time, even though its a zone they will still be matched up for a majority of the time, not sure how much of a majority but a majority.