I'm not sure about the 12 hour check through rule. I can say with certainly that maybe 2-3 years ago, I did SFO-EWR-YYZ, with ~ a 7 hour overnight layover in EWR - something like arriving at midnight and departing 7am, and the tag initially came out with both segments. It was a more senior agent there, along with a trainee, and the sr. said something to the junior about it being tagged through, even overnight, due to it being under 12 hours (or maybe 10), but that because it was overnight, she could manually short-check. Don't know if the same applies during the day.

I am pretty certain if you must pick up your bag in ORD, you won't be able to re-check right away, I believe UA will not take a bag at the desk more than 4 hours prior to flight. YMMV.

I assume your later flight is long-haul international in J? Hence the Polaris access?