My understanding (and we have no evidence of any of this) is that before the flight enters airport control (T-24), the waitlist has everyone in the order they requested an upgrade. So a P25K requesting when their window opens would be above a S100K requesting an hour after that.

However, once it enters airport control, the order resets, and is SOMETHING like status, then fare class, then check-in time. But previous (upgraded?) flying time on the trip may bump you up, and being nominated may bump you down.

As for your flight, they're either sold out in Y, or oversold. Y only reaches 0 when they are so oversold they know they're screwed. And I usually see that combined with a drastic decrease in J, since they know they're going to have to upgrade people.

You will almost certainly not get the upgrade in advance. You will be put on the waitlist, and you may get it at the gate. J7 doesn't look too bad, and it's possible they've lowered J availability because Y might be oversold.

Just keep checking the availability.