There are a series of dwarf Crepe Myrtles that might be lovely as well. If you just click on the Plant Database (upper left in the blue sidebar) and type in "Crepe Myrtle, dwarf" there are 6 or 7 different colors.
As far as the roots all over your yard, I think I'd pay a young, strong fellow to roto-till it for you. (rent a heavy-duty roto-tiller) Chop up those roots so they'll either be able to be collected and trashed, or they will break down underground much faster than if you just leave them and plant stuff over top.
Some really nice groundcovers will grow amongst the roots of trees pretty well, eg. Asiatic jasmine comes in lots of colors and variegations. You could just gradually clear pathways and have groundcover so you won't have anything you'll have to mow, or rake when your trees lose their leaves.
