Does the bucket brigade fill a pond only to 6 when filling from the level directly above? Why?
DFwiki says "Dwarves will stop
scheduling the Fill Pond job when the water depth reaches 6/7". To avoid overflows, probably. Filling from a cistern controlled by pressure plates allows 7/7 if you think it's necessary for some reason. Or just building a stair 1 level up and sending your bucked brigade there, but then you have to watch for overflow manually.
My current world apparently has no dwarven liaison! Could this have anything to do with why I haven't gotten any migrants despite nobody Dying Horribly?
Probably. On embark, DF warns you "Your selected civilization is dead or dying. Your dwarves might assume important positions." (I like it that nobles are officially as much of concern as, say, leaking aquifer or lack of fresh water
But you won't see it if other settlements are raided during the game, of course. Did you have a sudden wave of inherited titles?
I need yarn ASAP (my engraver is possessed and demands it) but whenever I tell the farmer's workshop to shear an animal, it says "no creature". I know that I have an ewe. I checked that ewe and it said that it has very long hair. Why will they not shear this sheep? Is it that her hair isn't long enough?
Never looked at this one, but probably a static quality rather than current status, so it just didn't grew wool after the last shearing yet. I'd also look out for workshops that may use up that yarn, BTW.
After a caravan had unloaded its goods on my trade depot, I received a message that I had no depot accessible for them. After then rushing to the depot, I found, much to my chagrin, that the depot and their wagons had been destroyed.
Look through combat reports. You may have a building destroyer critter around, possibly flying and still alive, just chased away, since you don't see a corpse.