What glue & screw/bolt/nut to use to fix the bathtub drain overflow?
By design, this assembly is not supposed to need any glue or caulking. The gray gasket on the overflow horn (visible in pic) is supposed to press and seal against the back of the tub wall, and the white gasket on the overflow strainer is supposed to press (no need to seal the front, it's just a cushion) against the front of the tub wall.
If it were me, I would:

remove the threaded post from the overflow horn
discard the original overflow strainer and cover plate
install a new cover plate using a new screw with the same thread as the old threaded post:


It is important to understand that the seal between the horn and the back of the tub wall is what is important, to keep any incidental splashing water (or, god forbid, an actual overflow) from leaking in the back, to the floor below. The presence of silicone caulk back there (visible in pic) is distressing because it could interfere with the gray gasket sealing against the back of the tub wall. I would try to remove that clear caulking as best I could...