I think you'll have problems with the 1/2" line with what you want to do.
Kohler has dissappointed me (and it appears others here), and I won't buy anything from them again. Their parts book could be confused as a phone book sitting there since they have the design of the day - parts are unlikely to be stocked, since they don't use the same thing - everybody thinks they can design it better, and they keep changing things.
A 1/2" valve can probably flow about 6 gallons per minute. Compare that to the specs of the heads you want to run simultaneously. A 3/4" valve could flow more (it's spec'ed to) if it is connected to at least 3/4" supply lines (can you replace back to where it is at least that size?). Picture your hose without a spray head on the end. You get some increased velocity when you restrict the opening...let it just come out of the open pipe with no restriction, and it won't go far. You may have enough heads to emulate that hose with nothing on the end.
Some of the Kohler stuff is VERY fussy about the rough-in - to about 1/16" or so or the body spray will either stick out too far, or won't tighten down and seal. You have to be really good about setting your tile and determining the position of the fittings.
For help on tiling, building the shower properly, and to verify the if the floors and structure are okay to support tiling, check out www.johnbridge.com.
Code requires the 2" drain for the shower, so you'll have to run it back until the waste is at least that size. It needs to be vented also...each drain needs its own vent line. they can be combined in the attic or in the wall above the flood plane of the highest thing on that run (probably the sink - must be 6" above the rim - inspectors often look for 42").
Moving the tub p-trap needs to happen, too. The only issue is maintain the 1/4" per foot slope from the outlet arm of the trap. If this puts it too high, then you've got problems, but probably not terrible.