Regarding 2: Shame to hear it, since I finally figured out how to make up + down stairwells, and now it's too late. Oh well: I'm sure my fortress won't survive too much longer anyway.
Regarding the answer to 3: Aha! Will check that directly. Thanks.
Regarding 4: Well, I thought I was going to need to irrigate, but discovered a huge underground source of sediment while digging towards the river. So I can skip the irrigation for the time being at least.
Little advice:
When last I played this game, it was horizontal. It seems as if being able to dig down levels has a profound influence on fortress architecture. Do you build vertically instead of horizontally these days? To me it seems like three decentralize floors of workrooms and two or three floors of living space built around a central stair shaft is absolutely the most effective design.
Is this the general consensus?