Welcome, Martin.
If you don't add that geographic location to your User Profile it'll be lost before we leave this page.
1. No.
2. I don't use AquaDefense and don't know to what extent it's expected to soften when wet. But the much bigger problem is that the AquaDefense shouldn't be on that floor surface at all.
You indicate that a new traditional pan liner was installed in, I presume, the existing clamping drain. That being the case, you really want the top mud bed completely open except for the tile surface so moisture that doesn't pass through the weep holes can evaporate through the surface.
3. There is no Liquid Nail product of which I'm aware that will bond a CBU - or anything else - effectively to your PVC liner. The mechanical fasteners are at least on the outside edge of the curb, but still unacceptable. If you were building a complete direct bonded waterproofing membrane shower, using CBU installed with mechanical fasteners at the curb would be fine. But you're not.
There is only one proper method of covering the liner on the curb of a traditional shower and that's to use metal lath and "fat mud," or brick mortar.
What you are doing/have done, replacing only the bottom foot or so of the shower, is considered a band-aid approach to repairing your failed shower. Regardless how the original shower walls were waterproofed, if they were, your new waterproofing membrane is not tied into that previous system at all. Your "new" shower may last a few years, but it will fail prematurely.
Up to you how you proceed, but what you really need is a new shower, properly constructed either by a competent professional or by you, a knowledgeable and trainable home owner.
My opinion; worth price charged.