Just another update here... I decided to withdraw the P+ request for the outbound flight only on the PNR that doesn't include me. Called UA to do this, but the agent misunderstood and removed the instrument from both outbound *and* return by mistake.

When I added it back to the return flight, I figured it would mean this other PNR would now be ranked below mine. But in fact, it returned to its exact prior position ahead of me in line! (All factors are equal, including fare class.) All I can think of is that it's still going off of time of request, and that when you cancel and then reinstate a request it sticks to the initial timestamp.

If the upgrades don't clear in advance, I'll test this theory at T-24 by checking in my reservation first to see if it jumps ahead.