Not to detract from any help you end up receiving on this board - but I highly recommend you post this as well on Slowtravel. If you're unfamiliar with that site, I think you'll find it a treasure trove of helpful info with very friendly people, and folks there primarily stay at rentals. There's also a STer who has a page on Accessible Rome which should be of use for your friend in the wheelchair during the Roman part of your trip.