I'd cut a piece of 3/4" ply about the width of the space between the rafters and about 30" long. That should go up in the attic between the 2by4s you were going to place between the rafters and the attic side of the plasterboard.
Before setting the ply and 2by4 piece in place, screw the ply to the 2by4s. Then smear a adhesive on the ply, on the side of the ply so the ply will be adhered to the damaged plasterboard.
Set the adhesive-coated ply in place. Use screws to secure the 2by4s to the rafters. If you nail the banging cuold cause more damage.
Now go into the living space and use the jack and the 2' square piece of ply you described to jack the damaged plasterboard up against the adhesive coated plywood that's in the attic.
Let it sit for 72 hours.
Should work fine, and it keeps you from having to repair screw holes.