Are drywall screws suitable for attaching a 3/4" tongue-and-groove plywood subfloor?
In this case, joists are 16" O.C. and no glue will be used.
By "drywall screws", I think you are meaning 1 5/8" blued screws. I do not think that they would be a good choice. We usually shoot down plywood subflooring with 2 1/2" to 3" ring shank nails, however 2" to 2 1/2" decking screws work well, but just a lot more work. I would encourage you to use a little adhesive on top of the floor joists or strips of felt paper or Grace ice/water shield to avoid any squeaks later on.
I wouldn't. Drywall screws should only be used for drywall, and a temporary coat hanger. You want more strength and need less space for the threads to secure the subfloor. The drywall screws have a lot of thread to hold the drywall itself, but have very little shear strength.
In the field between the joists, they are fine. As long as you use 3" construction screws to grab the joists and proper adhesives.