it's probably in the docs but...

[Andrez]

2) Yeah, Phantasy is right, you can't select both heightfields on top and bottom, that's because Cube is actually a 2D engine, and... Well it's a long story, but you can't, that's all.

1) I'm not sure what you meant here, but selecting a bunch of cubes, let's say 8x2, and Editing > Arches and slopes > one of the items, should do the trick...