I'm replacing a wall end threaded spout that was broken and didn't divert the water to the shower head anymore,

Upon removing the spout it came loose from the connection to the elbow and not the back of the spout, this revealed a galvanized threaded nipple heavily corroded.

I purchased a new spout and this time a brass nipple,

Problem is I can't get the brass nipple threaded very far into the elbow, the female threads have pieces of rust/corrosion including what I believe to be a piece of thread from the galvanized pipe.

Any ideas?

A ½" pipe tap, taped to socket, extension and ratchet, to clean the ell's threads.

just be glad that 98% of the nipple came out.
sometimes the entire threaded portion can remain behind.
you could try a copper fitting cleaning brush. one for 2/3 tubing should be the right size.
if the female threads are not too bad, it will work.
if they are too bad then as suggested use a pipe tap. the tap will hit the back of the elbow so you won't get the full benifit of it.
P/S, also be glad you have a drop eared elbow rather than one without ears. it saved the day..