As for the seat on a recumbent, that can vary. Mine (EZ Sport) is really cush. The back is mesh, and, hmm, my back doesn't even notice it. (I have a tender back, too.) FWIW, I've talked to Physical Therapists who have recommended 'bents for bad backs.

As you guessed, the range and variety of seats is wide and variable. I haven't been on one, but there are one-piece molded seats - but those seem to be favored by the go-fast crowd. The bicycle tourers tend to go for foam seat, mesh back. Most have a lumbar curve to the back, so are pretty ergo. As with all things, you really have to find out if your body likes that bike/seat. Keep in mind that many (most?) seats let you adjust the seat/back angle to your preference - and that that preference can change. They aren't hard to adjust. (Not available on hard shell seats.)

Test one and see what you think.

As for theft - all bets are off on a college campus. I like to think that my large and distinctive bike makes for a good theft deterrent all by itself. I lock it (generally) when going into a store, or similar, and I sure would if leaving it at a campsight while out exploring, but it gets noticed. Thieves don't like that much.

One last thing (Ha!), as of last night, the Bike Friday site has a used Bike Saturday up for sale. Something to consider...

One last, last thing (really), perhaps your Dad would "store" a recumbent for you while you are at college and you use a bike there that won't break your heart if it "accidently" leaves with someone else.