custom shower mortar bed question

Ask your tile distributor about liquid membranes. Most contractors are using this now. No need for any type of pan when these are used. You pour your concrete apply the membrane and the tile thinsets to the membrane. A much easier system. With a correctly installed pan, as you describe, there is no need to seal the concrete the pan is meant to hold any seepage.

Don't do it! If your vinyl liner is installed properly there is no need. Did you put a pre-slope in under the liner(give it some fall to the drain)? How far up the wall did you run the liner? It should go up higher than the curb. You should have a clamp type drain with the liner run under the top flange and clamped down tightly. You also should haveput some small gravel under the weep holes of the drain to keep the mud from clogging them.

Yes, I believe the liner is installed properly. I did everything you suggest including a 1/4" per ft sloped (to drain) mortar bed under the pan;

The sealer for the mortar bed would minimize the amount of water that actually gets to the liner and sort of be extra insurance for preventing future problems. My theory is the more water that goes down the drain over the tile, means less that hits the pan, creating less chance of perhaps clogging weep holes and/or eventually finding some other way past the liner. I guess I probably seem paranoid, but if you've ever had to do this job (removing a 3-sided glass shower door, tearing out tile floor, old mortar bed, lead liner and drain, remove tiles and durouck 2' up all walls, repair damaged subfloor, and then replace everything you just tore out including the ceiling in the room below) trust me, you will want to ensure you never have to do it again.

Thanks for the advice, Spud