I ripped out a 30 x 60 tub/shower and am replacing with a 34x 60 walk
in shower (prefab fiberglass pan). The plumbing is currently a 2"
drain that tees into a 1.5" tub drain and 1.5" tub overflow drain. My
plan is to cut off the t and replace with a 1.5" elbow connected to
the shower drain. Will this work and will there be any venting
problems? I.e. is the venting already handled by the existing plumbing
(which is below concrete slab I can't see).

thanks!!