Does the rear panel seal completely with the full roof on the 4 seater around the top? Yes, it seals very well at the top.
Would this, with the addition of half doors and windshield make for a pretty much airtight cab? Enough to stay warm with a heater? I have the glass windshield, though I only have the stock doors. The seal for the windshield is very good also.
Can the upper panel be removed easily for use in the summer? Yes, nothing is permanent, given some time, everything can be removed.
Does it hinder the rear passengers any? It shouldn't, the dump bed still has room to dump, though you'll have to have the window open to converse with the passengers.