bathtubfaucetplumbing
The pictured bathtub faucet pex hookup is not mounted, but I have back-wall access through a panel provided by the previous owner under a cabinet.
The faucet can be pulled several inches into the tub. Without removing the tub, what is the best way to mount this or a replacement fitting from the back for a professional and long term result?
Its likely the whole cartridge will need replacement. With it you will get a new handle stem.
This might be a sign to replace the whole unit. If you have a three handle setup, a cover plate will let you upgrade to any single handle style. If you go this route, be sure to get a tub filler spout WITH diverter (the single handle tub/shower valves usually don't have diverters.
Sounds like you need to replace the mixing valve. This is a simple part to replace. This usually involves removing the handle and possibly the trim, then removing a retaining clip, and the pulling out the cartridge. Buy a new one and reverse the process.
Here's a good set of instructions for Moen valves: http://www.moen.com/consumer-support/installation-help/tutorial?id=moen0133
Best Answer
Remove the incorrect fitting, replace with a drop-ear elbow, and mount a hunk of wood to mount the drop ear elbow to. This will be awkward and annoying to do (through that hole) but should be possible. You may (or will) need a longer hunk of pipe for the tub spout mount.
Other ways might be possible, but will be kludges, (like the "giant glob of construction adhesive" approach. Easier to do, but a definite kludge.)
Whoever did this install was an idiot. A tub only install needs one drop ear elbow, a tub/shower install needs two, they are made, and this is what they are made for.