As is common with new plantings by builders cpntractors, the maple tree behind my house was istalled with about a third of its root system above ground level in a too-shallow hole and with a token amount of soil to cover them. The planting was done about 5 years ago. My question: will it damage the tree too much if, using a sharp implement, the above-ground portion are removed from the trunk outwards and then replacing the mound with good soil for flower-planting purposes?

Maple roots are pretty much all in the first two feet. Any cutting will compromised the tree. May not kill it, but it will if you cut enough roots. Adding soil should have been done long ago and now you should only adding just enouh to cover the roots. Some Maple aren't very nice trees and their roots surface on their own as the tree grows. If it's a silver maple I'd get rid of it but that's just me. Mulching over the roots is fine.

I have a large Maple tree outside my townhouse ,in a very small area, can the roots go under the foundation,as it is they are well above the earth
Thank you for an answer