welcome from me.
1] I would assume a 0 degree angle of incidence with ||. XY would be 90 degrees and AB without looking at the literature, I assume would be 45-60 degrees
2/3] each of these are almost distinct types of sound: ambient sound, cat eating= maybe AB; car engine, music recording= maybe XY (for clarity- remember you are NOT making a super high quality music recording with these internals), city sounds - || maybe
the true answer to that type question is go out and do it, changing the patterns at specific time intervals(say 5 minutes), marking that down in a notebook, phone notes, and listening later to decide what your ears like best.
4] The self noise on these internals is a bit high to use without some post processing, or at least equalization/tone controls on your playback end. direct answer is: as high as the levels will tolerate for your source. (ie cat eating higher gain than car sounds- this due to the expected peaks to be higher with the latter)
5] not video guy. I generally don't do 96 as that implies large files to store. (or too large for me)
6] nomenclature is key from the start of a collection. Choose your determinants for folders- date, city, time of day, type of scene. I see many soundscape or effects products filenames labeled : church bells at 12 noon, or doors closing- elevator 1.... type thing.
I do not own a D100 but a friend does. He finds it tough for live music situations without external mics.