Well, the z-level dodging is a bug, so that will be fixed in time.
About your roof suggestion... It seems too unrealistic for me. True, some methods of building in DF aren't exactly lifelike, e.g. building a staircase that's only supported from above; but I'm fairly sure that this will be impossible once we have rope ladders in the game.
A different non-sensical building thing is that (almost) every building is constructed with only one stone block. That's a simplification, because hauling a realistic amount of stones to build a ramp would be extremely annoying. Fortresses would be dominated by huge piles of bricks, and blocks as they are are already bad enough for the FPS.
So, to summarize so far: in my experience, if some aspect of Dwarf Fortress isn't very realistic, it's either because a more authentic way hasn't been implemented yet (it's a placeholder) or because it wouldn't be fun to play it (it's a simplification).
As you said, building roofs from below isn't too realistic. In fact, it's pretty much unimaginable, especially for short dwarfs and their stubby arms. Let's see if it would improve the game as a simplification of a real-world method.
If you want to build a roof in Dwarf Fortress right now, you build a ramp/staircase and designate the floors.
If your suggestion were to be implemented, we'd just skip the ramp/staircase part and designate the roof. So, effectively, you wouldn't have to build a ramp and your dwarfs were less exposed and less likely to fall.
And that's the problem that I have with your suggestion: it sacrifices too much realism for too little improvement. Building a ramp is trivial; and removing it doesn't make the process significantly easier.