You need 2 credits each way. It upgrades based on Z availability, not P.

You earn 125% miles.

If there is no Z space, you will be waitlisted, and then it will be based on status, then fare class, if it doesn't clear before the gate. If it gets that far, you'd be low on the list.

I'd suggest you buy a Latitude flight pass if you can commit to five round trips in the next year. It upgrades into J space at T-48 hours, and works out to around $1000 each way, and is fully flexible.