Go which ever way you want on the farms. I am a farm fanatic, but hate fertilizer, hence my habit of building what many would consider to be too many farms.
As for digging out the adamantine, be careful

. We have enough going on above ground. Getting arms and armor and a properly trained militia would be good. However, 20 dwarves is a bit light for that, but I could see getting a good crew to train if you can spare them. Then if we ever get more migrants, we'll be set for splitting squads and training them up from the previous bunch.
Goblins do drown, trolls do not, at least not for a long time. I have used drowning traps on goblins before. Orcs, I am not sure. But if you can find a way to lure them into a trap, go for it. Or better, just dig a drop chute into the magma tube. I'm pretty sure the orcs won't survive swimming in that!