The sorting centers are completely different, since the methods of backbone transit are separate. Express only goes via airports (except for regional city pairs like L.A. to San Francisco). Home/Ground only goes by road/train, so you'd have to consolidate somewhere in between. Also, a significant fraction of the Express traffic is stuff that is time-sensitive delivery. Things like getting signed paperwork so people can buy houses. They really hate delays. In fact, if you spring for the "priority overnight" they'll send a driver to your door with a blank waybill and envelope, and wait while you fill it out. It's quite a different service than Ground/Home.

Depends on how much time/trouble it would be to consolidate in time for that last mile. You could expend more effort combining things, than you would save in just doing the last mile in separate trucks.

I'm not saying this IS true, I don't know. Just that it is logically possible. Combining two different systems for a last mile delivery is not trivial.

No idea. UPS doesn't work that way. As a small biz, I went with UPS because I didn't want to deal with separate air and ground pickups.

It might make it easier to deal with FedEx packages that have to be delivered really early, early, and the like.