Hi Emil,
I had a sudden inspiration to open this thread.
I looked over the last few pages to catch up, but forgive me if I cover things that have already been discussed.
Pre-slope
The photos of the pre-slope concern me, but I believe you've already been made aware of this. Typically, I will use the bottom 2x4 as a guide. I would guess that you are shooting for 3/4" of slope (from the edge to the drain)? If so, factoring in the bottom flange of the drain is 1/4" you will want to be at about one inch high around the perimeter, in other words, you will see the top 1/2" of the bottom 2x4 all the way around. I hope I've explained this well.
Liner
If I were hired on to install the liner in your shower, I would not cut the folds going over the curb. The only punctures that I would have in the shower area would be 1) the drain hole 2) the four bolt holes. All other punctures would be A)the top of the liner, at least 6" above curb and B) the front of the curb
outside the wet area-not the inside or top of the curb.
Thanks to Ceci, I took photos of a liner I did so people could see how I fold them. The pictures make it much easier to understand. The liners that I buy are black 30mil liners from Pasco. It looks like you have a 40mil, so it will be a little more difficult to fold.