Sounds like a fun trip. More power to your mom!

I've done the Cumberland to DC ride twice, once in 5 days and once in 3. Where you stay will depend on your mileage of course, but I would recommend any of the places I've stayed.

Cumberland--Holiday Inn
Little Orleans--Little Orleans Lodge
Hancock--1848 Trail Inn
Shepherdstown--Red Roof Inn (can do laundry here)
Harpers Ferry--Towns Inn
Leesburg--Best Western (ferry across the river and they pick you up)

As for transportation, that's always been the challenge. Other than renting a car one way or having a shuttle driver accompany you, I've got no ideas. I seem to remember that there was a shuttle service between Cumberland and the northern end, but I don't know anything about it. Maybe the Amtrak option will be available, but who knows when.

Have fun!