Shower pan inspection

No I would not apply water proofer to the subfloor under the shower pan (unless we are going to be waterproofing the entire floor it might be easier to just continue it under the shower pan).

I think The Evil Greebo does bring up a good point, because this is a steam shower I would be even more concerned about mold developing in this humid environment. IMHO, if you will be tiling the floor I would put a water proof membrane under the entire floor and not worry about using pressure treated wood for the subfloor. If you use the DITRA and cover the seems that should be waterproof so there really should not be moisture getting to the subfloor.

Of course the (relatively) small increase in price for treated plywood may be worth the peace of mind (are be a backup if water makes it through the floor's water proofer.

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...)