To answer your first question, If at all possible you should secure the wall unit to the tub in the holes that secured the unit when it was shipped. First you need to run a bead of Adheasive Caulking to the tub where the fibergalss wall will sit. Make sure you use a water based caulking for this, as a silicone will be hard for you to clean up if any gets on your tub. The way we usally put the wall is to do it in the bathroom and then slide it in place. a skinny person might be great if you have a tight bathroom! After the tub is in place, drill a couple of pilot holes in the upper flange and secure it the the framing with Galvinized screws.
The best way to drill your holes in the fiberglass unit is to very carefully measure the location of the faucet handles from the wall and transfer this to the BACK of the shower unit, Measure it a couple of time to ensure you got it right, it helps to mark this measurement with a peice of white caulk. Drill a small pilot hole from the BACK of the unit first. Next you want to take a HOLE SAW, and drill from the front of the unit, this ensures a nice clean cut in the fiberglass. You can use Thin Set, or bonding morter to put under the tub. The reasoning behind this is to fill any voids under the tub you might have after leveling it, and to create a solid base for the tub to sit on, Fiberglass tubs do flex, and without mud under them, after time are likely to "spiderweb" on you, (little cracks) This usually requires about 2 inches of mud under the tub, you install the tub while the mud is wet, level the tub and let it set up. After it dries you sould have a solid floor for you new tub. Hope this help you out......
Simon Home Improvements Inc.