To avoid confusion, use your UA FF details. However if you want to start earning miles & points with Copa, then use that

You will get lounge access at each airport due to *G. Extra bag too for the same reason

Next to 0 chance for any comp upgrades, no matter what the fare is

You can choose most economy seats for free ("premium" seats in the first 3 rows of economy and exit row seats are not free). If its the MAX9, then you will have slightly more restrictions for seat selection, but will still be able to choose some for free