I am planning to visit PTY in January and have a booking question. I plan to book directly on Copa JFK to PTY which looks to be a Copa operated flight.

But there is a return flight from PTY to EWR that is much cheaper than other flights and looks to be operated by United instead of Copa (other flights for more $$ are PTY-JFK).

I can book with Copa for $587 and with United it's $730 (Economy). The other returns from PTY to JFK are all over $1k (operated by Copa I believe) ...
The date is 1/15/23 for reference PTY to EWR / JFK. I'd much prefer JFK but fine with EWR at half the cost.

My questions below ...

- Any issues with booking on Copa (to get the better rate) even though this return is operated by United?

- I also want to upgrade my seat to Economy Plus but when booking on Copa it says Unavailable to make seat selections during booking process...once I book, will the flight and upgrade availability show up on my United account? I assume I need to link my Copa acct with United number?

- Anyone book this route and have any experience or advice?