I can try to answer your first question. I do a lot of bottle conditioned beers, and although I don't drink, nor brew IPAs anymore ( except maybe once or twice a year ), I still try to avoid O2 as much as possible. If you are following an established/well working fermentation schedule, your beer is definitely finished fermenting at day 14. You shouldn't really need to open your fermenter and check anything. Diacetyl should be taken care of by day 14 and your beer would've reached FG. If you are cropping the yeast, then you would do that during fermentation, or at least is what I would do. Any O2 that comes in at this step, will certainly be scrubbed out. Also, I will add that once you get to know certain yeast and how they ferment/behave, the necessity of taking gravity samples all the time, will slowly disappear. A 1.050-1.065 beer will never need more than 10-15 days in the fermenter.
I personally only open my fermenter once, and that is bottling day ( except beers I add adjuncts to, like vanilla, oak chips, cocoa beans, etc. ). At that point I take a gravity sample and I bottle. So the beer comes in contact with O2 only one time. It will referment in the bottle and some O2 will go away, or at least is what I like to think. Dark, complex beers do not take damage over time, not even after 9-10 months, so I must be doing something right most times.
Cold crashing can introduce O2 in the beer, unless you take meaures against it. Cold crashing works great with all styles of beer, and I feel it helps beer to become " clearer ", " better " and " more focused ". I am not a stickler for brilliant clear beer, but some styles do look prettier with a certain degree of clarity, especially those with pretty colours, like orange, deep orange, amber, light and dark red, etc.