phylhl-- Let me guess-- you're in Mass.? Copper pans, are, for all intents and purposes, outlawed just about every place else. The only reason they're still allowed in Mass. is because the plumbers union makes alot of racket to KEEP them legal. They've been outlawed elsewhere for three reasons. First, they're usually flat bottomed, and as TileTech alluded to, the pan membrane must be sloped so as to make sure all water that gets into the pan goes to the weep holes. Which brings me to point 2-- there are NO WEEPHOLES in copper pans. Any water that gets into the pan stays there, making for the perfect breeding ground for mold, mildew, and bacteria. Lastly, over time, the lime in the cement used to form the shower pan will corrode the copper, and I don't have to tell you the mess that'll make.

First thing I'd do is read the installation instructions to see what is specified there. If they recommend "X" number if vertical inches and you're less than that, or if you used PEX and they specify "no pex" or you used pex that has a smaller inside diameter than the recommended tubing, then essentially you have an installation issue. 1/2" pex does have a smaller inside diameter than 1/2" copper. For delivering water volume to a faucet it's normally not a problem, but when depending on a specified vertical water column, I guess the smaller diameter of the pex could be an issue. I'd call the manufacturer's tech support line and ask them about remediation. No harm no foul, it's best to run ideas past them before you crack open walls. You installed the shower head lower for the kids. Let's say you have yours 25" above the valve and the manufacturer specifies 30". What you could do is have the pipe come vertically out of the valve, and run it up the wall several inches higher than the shower head, say to 40", or even all the way to the top plate of the wall, then add a couple of 90's (to create and inverted "U") and then run it back down to the shower head. That would take care of the vertical issue while allowing you to still have a lowered shower head. If your pex to the tub spout is constricting flow out of the tub spout, that might need to be changed to a larger size of PEX or to copper. Something with a larger inside diameter. You're on your own well so it's likely excessive water pressure isn't an issue. What's on the "back" side of the wall that the valve and plumbing is installed in? Often times it's easier to cut through the back of the wall to access the plumbing, repair the plumbing, then TEST the plumbing, then patch the hole, than it is to access through the shower side of the wall.

Yes, all you need is the shower valve, a length of copper 1/2" pipe going vertical to an eared transition elbow from that shower valve, a shower arm screwed into that elbow and the hand-held shower wand screwed onto the shower arm. The PEX pipe coming from the house will attach to transition fittings soldered to the mixing valve. You can use individual 1/4 turn frost-type sillcocks coming through the house wall to supply hot and cold to the PEX if you wish but these just complicate the transition to the PEX. I would just install 1/2" ball-type valves with bleed screws on them, inside the house and then come downward and then out through the house wall with copper to a 1/2" copper coupling. That way, the coupling can be undone at the end of the season and the irrigation guy can blow out each line with ease. Just open the bleed screws on the valves inside the house and any water in the pipe after the valve will drain out via gravity. The lines coming from those shutoff valves inside the house must be installed so that they will self-drain via gravity if possible. If not, then you will have to stick a pot under the bleed screws when you open them up because the water still in the pipe will flow out of them instead of going the other way. If the couplings are installed so that the hot is different than the cold, then the hot/cold won't be connected incorrectly in the spring. If you look at a coupling, you will understand what I mean because the rotating nut is on one side of the coupling unit. Have him flip one of the couplings opposite to the other. The Moen sillcock unit mentioned by redwood won't do you any good if your shower unit isn't in the same location. That sillcock is a hot/cold mixing valve. It might be kind of em-bare-assing to run from the shower to wherever the sillcock is, in order to adjust the water temp. Also, there would be no point in having a shower valve in the shower stall because that sillcock delivers mixed hot/cold water through a single line. The single handle mixing valve requires only one large hole vs two smaller holes for the valve with individual hot/cold taps. Don't worry about what's good for the irrigation dude. Choose what's best for your needs. If I was doing this shower for my family, I would choose a single-handle mixing valve, preferably Moen.

Yes, the galvanized pipe is probably a 'placeholder' for the drywallers to make a hole for. Then when the finish shower trim is installed, the galvanized pipe is removed and the chrome shower neck is installed. Sounds like SOP.