Putting a shower stall on a platform is considered to be quite tacky. You should consider that you will have to live with this for as long as you live in the house. All drains must have there own trap, so yes, the shower must have on. Also it must be vented. The drain must be 2" and have a 1/4" per foot slope. This means cutting a hole in the slab to accomodate the trap. The shower drain has to be centered on the stall.