As for your hypothetical and the "possession" of a pre-ban mag >10rd at the range...
Since the RSOs are not checking firearm registrations, they have no idea who owns what nor who is on the registration (registration does not necessarily equate to ownership, and HPD would have a fit if we went down that road). If I say someone stole my gun -- and I have the receipt from when I bought it -- the person on the registration would also have to produce proof of ownership if they have it. People bring guns in all the time to register with no bill of sale (out of state arrivals). As long as it wasn't listed as stolen in a database, the gun gets registered. So, registration is in no way proof of ownership.
Anyway, if I can hand someone a loaded firearm at the range to shoot, and there's no attempt to ascertain which of us is the owner, and for that matter whether the person being handed the weapon is prohibited from possessing guns, I don't think a magazine that might be blocked to 10 rds (can't tell just by looking from a distance) will be high on their list of laws to investigate.
There are two sides to the coin: what is lawful, and what is 99% unlikely to be reported. This bill, like most so-called "firearms violations" would be used either to pile onto other crimes they are investigating, or in place of statutes for which they were unable to find sufficient evidence to charge you.
In short, the whole "what about at the range?" debate is a special case. As I said, if you're at the bullseye range, you aren't allowed to load more than 5 rds in a mag at once anyway. If you feel strongly enough to want to take pre-ban mags to shoot and not have 10 rd or smaller mags also, that's up to you 100%. While you might be technically in violation of the law when you hand someone your handgun, or a rifle with a pre-ban mag, I'm confident nobody will be waiting to pounce.