Shower – can i use a mortar shower pan before its tiled

Apply thinset to low spots and ride off high spots with a straightedge to even the floor. A shower floor is not something you want to do over a hard mud surface. The preferred method is to pull your mud and lay your tile fresh, allowing you to use a beating block (4x6 inch is more than enough) using the block and riding at an angle to the tile beat the floor flat.

The biggest mistake pre-floating the mud is that the drain can be an issue if you cut your mud too high in relation. I should note that over fresh screed you want your thinset very wet as too stiff will pull up patches of mud if you pull a tile. If you do have to pull a tile when fresh, slide it while using a lifting motion.

I think the fact that a "dark damp patch appears on the concrete" when you open the shower valve(s) yet catch the water in a bucket points directly to a leak from the plumbing. You need to open the wall from behind to gain a view of the shower valve body, and also remove all trim on the inside. Use the handles to open the valves and find the leak source. Water may be running down the interior of the enclosure wall. The fact that you have a plumbing leak does not mean that your shower pan is in good shape; if it is old, since you have removed the tile, this is an appropriate time to break out the concrete and have the pan refurbished (hot-mop, vinyl liner, fiberglass pan, etc...)