tiling ect

hi , ive started a bathroom remodel, ive pulled up the old mdf floor (part goes under wall into bedroom)
ive pulled off the plasterboard (to get tiles off) and installing a shower base.

my question is, ive a rim of about 1.5 inch around the area where shower is to go ( this goes under wall into the hallway (supporting wall), and about 3 inches at the thresh hold.

ive got 18mm wbp ply to do the bathroom in, do i have to try support this wall to try cut away wood or can i just tie onto this bit?

also where the tray is going to go theres a lip of 1 mm, will adhesive or grips all be ok to use and wlil it fill the raised area (theres no instructions with shower tray to say what to use its stone resin)

thank you