Also, is it possible that if you melt the trench and then channel the ice floor once, it will melt/freeze properly (like natural water in a season changing biome) next time?
I tried that, when I dig up the floor to expose the liquid water one z-level down, that water refreezes right away. When it does so, it creates a new floor on the surface level where my dwarves just dug from. Then the new floor still won't freeze from magma below.
From what I witness, when water (5/7 or more) freezes, it creates three things: a floor on the same zlevel (if there is no constructed floor there already), a wall on the same zlevel directly on top of that floor, and another floor one zlevel up. It's that one up floor that is giving me the trouble. While I need that floor to be there for the enemy to stand on, I can't melt it from two zlevels down.
Sometimes water doesn't create that floor above (not sure what prompts that), but that isn't what I want either. That would cut off the area from invaders since they can't traverse open space.
I know I can melt a floor (even with no ice wall on top of it) by putting magma one zlevel up from it. Melting diagonally doesn't seem to work. For the purposes of melting and warm walls, temperature only travels one square in any direction, where diagonals count as two squares away.