I usually finish such a drywall edge with a j-bead and then mount?the?plywood next to?the already finished edge. No way (and no?point really) of hiding a joint between two such vastly different materials.?
As for your second question, it depends on many factors - what thickness?ply, what is the surface area of your ply, what is your ceiling framework (metal stud, wood stud, concrete slab), etc.