For the plywood, use a circular saw set to the right depth along with a hammer and chisel for the ends. Or a rotozip tool may do it.
The edging sounds like drywall corner bead. cut with a hack saw and it is usually nailed but newer may be screwed. If the corner is to remain without messing up the face too bad, then just chip off the drywall mud from the tub side of the bead, remove the nails and pull out the old drywall and insert the new.