Return it for a real couch and buy a blow up Aero bed for sleep overs. : )

There really isn't a sleep sofa that feels like a regular sofa. You use these in rooms where the couch need not BE as comfy as your typical family room sofa. A study, perhaps.

Otherwise? Lay some foam on there and buy a ready made stretch to fit slipcover for the entire piece. Not great as both will have to come off for sleeping. Your idea? No.....will turn to ugly: )

Keep a stack of 24 inch square toss pillows with down fill, and plop em on when you sit. Contrasting color? Who cares. Don't keep them on the sofa full time.

You will need to get a new sofa. There is no way you can retrofit as you are proposing. Sorry. Especially if you are trying to rent this property out and advertising the sofa as a bed. And, the same with a blow up bed. I don’t think a renter will appreciate that at all.