I was in Rome 6 weeks ago and went to see the Sistine Chapel. We booked timed tickets into the Vatican Museum, which does NOT get you into the Sistine Chapel at a certain time, it just means that you can bypass any queues to get into the Museums.
http://biglietteriamu...eTipoVisita=26&step=2
I would say that it IS a long walk through the Museum, we took the 'short route' to the Sistine Chapel (at least those were the signs that we followed), and it still took us the best part of an hour to get there (though I suppose we did stop and look at things on the way). Yes, I think that you liking it to IKEA is just right, you know what you want to look at, but have to go the long way round! There was a lot of Museum that we did bypass by following the 'short route', eg the Egyptian bit, which I wouldn't have minded seeing, but we didn't have time to do everything.
I've just had another look at the Vatican Museum website and you can hire wheelchairs, which may suit your visit
http://mv.vatican.va/.../MV_Info_Servizi.html