You have a lot of threads to read up on to get the answers to every question you have. In a nutshell (and Google search "Flyertalk United xxxxx" to get the exact threads):

- United will oversell Premium Plus on just about every route. When they do this they will block Polaris seats to cover for those who don't have a seat in PremPlus. They never downgrade them to economy but rather OpUp them to Polaris...IF needed. But usually upgrade requests using PlusPionts from PremPlus to Polaris will cover them just fine.
- Zeroed out means the flight is J0O0Y0 meaning United will not sell a single seat in ANY cabin on that flight
- The web site is notoriously bad at displaying the upgrade list info. The capacity and checked-in numbers are the only thing that's correct. The "booked" number is wrong basically 100% of the time. Do not use this as any measure for the flight.
- The seat map is always up to date but does not take into account those who are booked in the cabin but haven't selected a seat. Or, as you likely figured out, the four seats you show open relate to the four seats United has blocked for the 4 oversold in PremPlus.
- The mobile app will show "Booked: Full" whenever the cabin is literally booked full. Even though it's likely booked to 42 + 4 blocked the app won't display that if it's full.

Your chances of an upgrade...nearly 0% unless you are already confirmed in PremPlus and your PlusPoints are applied correctly. But I assure you...the system is not skipping over you and upgrading others as United process zero upgrades on that flight given the loads.

PS - I could be wrong but I think Skip The Waitlist PPs are null and void if you voluntarily change your flight. The agent took out 30 more PPs because the others became "expired" when you changed. So 30 PPs tells me you might be in PremPlus already which greatly increases your chances of an upgrade but they won't process until the gate.

-RM