Why is there malted barley flour in all purpose flour?
Note - label on my bag says all purpose flour, not all purpose wheat flour. So you can take that strawman (#4) for a long walk on a short pier. You won't find it in the "100% whole wheat flour"

Purpose (#2) - malted barley is yummy yeast food (or the makings of beer.) So, not surprising that you find in in flour intended to serve the needs of yeast baked products.

What is it? (#1) Barley (a grain) that has been malted (sprouted) which converts starches to sugars by enzymatic action, and is then kilned (dried) to stop the process, then ground into flour.

Percentage (#3) no idea, but likely closer to 1 than 30.