Yes, it clears into PZ.

If UA goes by the published order, they're supposed to upgrade paid PE to J before anyone in Y goes to J, regardless of status. So, you should be ahead of 1K passengers trying to use a GPU to go from Y->J, but you'd be behind a 1K on a paid R fare. (I'm not suggesting that they're not going by this order, just that the upgrade lists have looked really wonky since they introduced PE).

The number of seats on the seat map doesn't really matter. Use the future flight status tool: https://cablepick.net/united .

ZN6 P0 isn't great, but it's not awful either. What day of the week is the flight?