Anthony wrote:

It all started when I forgot about removing the set screw, before I
removed the old spout.

The copper nipple's threads no longer exist...therefore, I had to
remove the nipple...using a pipe wrench, I still could not get enough
torque to break loose the nipple...I ended up partially twisting the
nipple...

Any suggestions???

Set-screw spouts are slip-on; they don't use threads.
This is plain ole copper pipe (tubing) you have.
Very likely, the end inside the wall is not threaded into
anything, but rather is soldered into an Elbow (not visible).

First step would be to gain access to the space behind the tub.
Normally, there will be an access panel. If not, can you make
an opening in the wall on the other side?

If it is as I'm imagining it, you will have to either solder
(torch) a new length of copper tubing in place or, maybe,
cut the old tubing and use a compression fitting coupling/elbow
to avoid soldering inside the wall.

Sorry to say-experience counts for a lot in difficult situations
like this one.

Jim