I would get a live-reading OBDII reader and watch the two oxygen sensors in real time. The voltages fluctuate from 0-1, and if the cat is bad, each sensor will show about the same fluctuation in voltage. I'd also expect to see a PO420 to show the bad cat. You can purchase a OBD tester that shows live data on Amazon for about $140.
You can move the 'stuck injector' to another cylinder and see if the code follows it, but this is sort of a pain messing with o-rings and fuel rails.
I think you're on a good track to eliminate any/all vacuum leaks - I'd rate that as a higher likelihood right now.
Colo Springs, CO