I recently pulled 3 10 ga wires through 1/2 EMT and it was far from a
walk in the park. And that was out in the open pulling one section at a
time then joining the sections. Not bad on the straight parts, but a
bear through the 90s. I wasn't using stranded wire so that would help.
I don't think it would have been possible with small wires already in
place. As to legal, current thinking is that you need 4 wires for 240v
I'd imagine 3 would suffice for 120v.