I had some plastic Promag mags that worked well at first, but eventually started causing malfunctions. Switched back to USSRGI steel and no issues. I would stick with them.
As for the legal issue, no, you wouldn't have any legal liability as long as you didn't assemble the rifle. 922(r) only prohibits assembly of the rifle in a non-compliant configuration, so even if it doesn't have the requisite number of domestic parts, its not on you. How are you supposed to know the country of origin on all the parts if you just bought the thing, anyway?