Originally Posted by hammerlane
Then I would forget about soldering all back in there and buy a brass coupling(5) and a brass nipple(4) if you have the depth and put it all back like that
A reminder inspired by your specific pictures. The OP and anyone eavesdropping needs to make sure that the pipe threads match.
Never force fit non-matching threads even if you use plenty of pipe dope or Teflon tape.
It is okay to make up the entire shower arm assembly first and then screw it into the elbow inside the wall using the angled down portion of the arm assembly to crank it into place, no socket wrench needed.
Regarding leak testing:
If there is any play around the arm pipe where it exits the wall, you could turn it on for several minutes and shut off the water flow using a sprayer attachment on the shower arm, then peek inside with a flashlight to check for drips while it is under pressure this way.