1) I believe it's 24 hours but am not sure. If you want a stopover within the U.S., note that AS only allows stopovers at the international gateway--in your case, SFO--where you transition from AS to the partner carrier. Other stopovers at en-route international cities are dependent on the agreement AS has with the partner carrier.
2) I think I've heard of this working but am not sure. Best to ask AS or wait for someone with more knowledge to post here.
3) I haven't personally paid attention but my recollection of others' posts on FT indicates it is as you say--some seats being opened extremely far in advance and then some more seats being opened within a week or two of departure.
4) The change is on any change whatsoever. Date changes are not exempt. (AS does not discriminate between "simple changes" or "complex changes" or anything like that.) The one exception is that change fees are waived for AS MVPGs.