How can I repair a nicked wire without adding a junction box?
I hate to leave questionable connections behind walls. I dream of fire and destruction.

I would repair the tear unless you were certain that the wire insulation was not compromised.

The easiest way to do that is with an inline cable splice.



These are mean to be concealed in walls where repairs to existing wiring is needed, and are UL approved. I use them sparingly (they are a hidden connection) but sometimes they are necessary.

Yon need some slack, so you may have to open a bay next to the current one to get more cable. You can then run a new cable to the next box. In a pinch, you could use two splices and a short run of cable between them, but I would try to avoid that.