for basement thing, you can split your terrain into 4 parts and leave a hole there.

also i am guessing 3 can be achieced with a custom shader?

These limitations(rectangular,cannot be rotated, all vertices has same space between them) are the point of the terrain engine, by having these limitations, it can make a lot of assumptions instead of calculations which makes things a lot faster. If these limitataions were to be removed, they would not be much different than models and that would not make any sense then.