(1) The Brewing Network explored that question, and decided that 100IBUs (approximately) is the limit of the amount of alpha-acids you can dissolve into solution. I don't really have an opinion, but that's their claim.
(2) I understand your question, and it's clever. I'm certain that the answer is yes, since I have made partial boil IPAs with much higher than 50IBUs in the past. That - for me - casts doubt on the claim made by TBN!
(3) Leaving break material in the fermenter is essentially like dry-hopping, except your using spent hops. It might add some aroma, but not much.