“Beyond that, I wonder what would happen to a positively charged black hole, if you started pouring electrons into it. Would the event horizon still grow?”
It should since it’s proportional to the mass and by adding electrons you’re increasing the mass.
At this point, I’ll resort to argumentum ad verecundiam, based on an article that I just read:
“Together, the black hole’s mass and charge determine its size — the radius of the event horizon.”
There’s also some other interesting discussion about shrinking, charged black holes.