Plumbing – Soldered copper vs threaded copper at shower valve

Pros:

Glue up

• wall is flatter, less variance, less warping
• generally easier to install (once prepped/drywall up)
• if going drywall underneath less echo/tin can sound
• on a direct install you must be able to pack the cavity with insulation to prevent push-in and (more) echoes, glue up you don't worry about that
• transition to other parts of the bathroom is easy since drywall to drywall or drywall to backer is really no transition

Direct

• Can have it installed very quickly (no prep)
• don't have to worry about water affecting what is behind it
• the more expensive ones have good transition molding that allows you to install drywall after with small seam to be caulked.

I'm no plumber, but here's a thought...

Compress a copper nipple onto the shower valve. Now instead of the adapter you made, install one of these into the wall:

http://www.screwfix.com/p/male-coupler-15mm-x/82086

Now, place the other end of the nipple (already attached to the shower valve) into the compressed end of that fitting, and to lock down the nipple you will be tightening in the same direction as the threads going into the wall. So basically you end up with a compression fitting on both ends of the nipple.