I know it's heresy here, but I'd abandon the mud bed. It's old school and there are so much more foolproof modern methods available, and that mud bed is what's causing your detailing problems.

There's Tile-Redi, Wedi, and others. With Schluter, you can use drywall in your shower. That's waterproof. That's detailing. That's 21st century.

If you're going to use a membrane on the Durock, and if you're concerned about trapped moisture, then I'd recommend you look at Laticrete's Hydroban membrane.

It's a great membrane for waterproofing a shower. But it's also vapor-permeable.

If you're interested in going that route, then I highly recommend you abandon the plan for a vinyl liner in the floor.

Install your Durock on the walls.

Buy a Kerdi flanged Drain or Laticrete's Hydroban Flanged drain.

Set the drain and do your shower floor of sloped mud.

Build your curb.

Then Hydroban everything; the shower walls, the sloped mud floor, and the curb.

My husband spoke with the tile guy yesterday. He uses Lacticrete/epoxy grout which he felt was going to work as form of waterproofing in itself to keep water from penetrating behind tile--but is willing to use liquid waterproofing medium over the Durock. He also said that his bid did not include building the pan since the plumbers have to jack current slab and relocate drain from its current location (for the original tub) into middle of new floor space for shower...the plumbers are supposed to build the pan with preslope... he said they/plumbers would put in vinyl liner and a conventional drain....These two guys have done many projects together before and guess they know how the other operates.

We didn't see anything in plumber's bid about building the shower pan so we have asked for clarification but no answer yet. I don't have problem using the waterproofing liquid over the pan vs vinyls liner...What is price difference in using Kerdi or Lacticrete drain over conventional?

The tile guy told my husband that the vinyl liner should go up the shower sides about 10 inches and then be tucked behind the Durock....(if my husband heard right)... Is that approved practice? I thought there was supposed to be gap between those two surfaces that was filled with silicone--that they didn't touch...am I mistaken? In case they decide to go that route...

Should/can the exposed ends of the Durock be waterproofed?

If you are in Tampa, I know a really good tile guy. He tiled two bathrooms for me and he is really good.

"The tile guy told my husband that the vinyl liner should go up the
shower sides about 10 inches and then be tucked behind the Durock....(if
my husband heard right)... Is that approved practice?"

That is correct.

If the vinyl liner is the way you want to go, have them put in a mud preslope. Then cover that sloped surface with the liner, with the liner going up the walls about 10".Now your liner is sloped to the drain, which is a code requirement. It's a violation to have the membrane flat on the floor. It needs to be sloped.

The wall framing and the liner that wrapped up the wall gets covered with 1/2" cement board.

Another layer of mud goes on top of the sloped membrane.

"...but is willing to use liquid waterproofing medium over the Durock."

I recommend you do that. You can Hydroban Liquid on the walls for your waterproofing. You can use it on the mud floor too, though the area at the drain will have to be properly detailed. If they don't know how to do that, then just use the Hydroban on the walls.

Then tile.

I don't see the need for an epoxy grout, but if that's what you want, you can go that route.

If the installer states using epoxy grout as his waterproofing is reason to keep a close eye on what is done and take photo's of the process. Liner height over pre pitch would be a minimum of 3" above the finished curb.

No grout lists itself as 'waterproof'

Did you use ceramictec, by chance, Jerzeegirl?

It's amazing how many "tile guys" out there think the "right" grout and caulking will take care of things in a shower. In a properly done shower, whatever method you choose, the tile and grout is just the pretty covering. All the waterproofing is below the surface.

Personally, I'm partial to sheet membrane systems, of which there are many these days. There's Kerdi, Noble, Durock, Redgard, Hydroban and I'm sure others I'm forgetting. They do the best job of managing water, IMO. In regard to pvc liners, my view is that a horse and buggy will still get you where you need to go, but a car is much more efficient. :^)