Second myth: Constructing a roof above a floor where crops can grow makes it possible for underground crops to grow there.
Haven't done any tests, but since you're basically describing a greenhouse, which is used to grow aboveground crops, I'd assume that this doesn't work.
Also, I can confirm that magma doesn't flow off map edges. My test wasn't nearly as dramatic as the one already shown, basically just consisting of a tunnel to the edge of the map formed by raised drawbridges, with a screwpump that filled it with magma. When I turned the screwpump off and waited, the tiles stayed at 7/7. Well, it might not have been 7/7... I forget. The point is though, it was consistent, with no magma flowing off the map. This was above ground.
As a suggestion... How about turning magma into obsidian via bucket-brigade? I heard somebody say that it didn't work, but I'm not sure how full the magma tile was... I think magma only turns into obsidian once it reaches 1 or 2/7 before coming into contact with water, with any water added before that turning the water into steam and making some magma disappear. Not positive though. That might be worth looking into as well.
Then there's the artificial chasm rumor... Looking at the thread about it, it would seem my best bet would be to collapse some ice on the bottom z-level, then melt it. If that doesn't work, I can try to take advantage of those bugs which turn random tiles into empty space, then try dropping things in them. I'll post any results I might get.