I was thinking of using a liquid system over CBU and utilizing a kerdi drain assembly. Three questions: 1) I have plywood on all walls (to stiffen 10 foot 2x4 walls in shower) - is there a problem with installing CBU right over plywood as opposed to studs? (maybe with moisture build up due to no air space between wood and cement board?). 2) I know the industry standard for a wood sub floor is to put plastic down with stretch metal under the deck mud for the shower floor base. But, with the liquid membrane at the top, does this create a vapor sandwich? 3) I noticed no one uses red guard. Is there a reason related to performance or is it just a cost issue?

Plywood is fine without a poly vapor barrier under the CBU as long as the CBU has a waterproof coating applied to the surface.

A shower pan with a topical waterproof coating will not create a vapor sandwich. You only need enough protection on the shower pan floor to help with mortar curing so that the bare wood doesn't suck the little moisture that's in the mud mix out.
You can use tar paper or 4 mil poly. You don't need 6 mil poly on the floor.

In my area, RedGard is only sold at Home Depot in the small pails for the DIY homeowner market. Most tile installers buy the larger 20L pails at wholesale prices from Laticrete or Mapei dealers.
It's more cost effective for me personally to buy the larger quantity.

After using them all, my number one choice for liquid is Laticrete HydroBan.