...
I recall once when flying United, we had booked all four of us on one ticket, though my wife and I both had Explorer cards. The check in agent insisted on charging us for our third checked bag since technically only two were free on the one card. Later we were able to get this refunded though via Customer Care. ...
Not sure I understand this, the Explorer card provides
first bag free for the cardholder that used the card to purchase the ticket and first bag for one companion. Even if there was a second cardholder, since their card was not used, you would have not been eligible for additionally bag fee waiver. You got luckly and doubt it would be repeated. best to read the policy
Free checked bags for eligible United Cardmembers
and the FT thread
Mileage Plus Branded Credit Card and Checked Baggage Fees
... Do you think something similar might work in this situation; namely if Avianca charge us for bags, United may reimburse us? ...
IMO, doubtful but you can always try
... The e-ticket does indicate free bags for all flights, including Avianca/Taca operated sectors. . . .
...
This is a unclear area as the immediately below that table it says something like "Baggage check-in
must occur with United or United Express, .."
However some have argued the DOT requires correct bag info on the receipt.
Personally I would expect to pay the normal ticket bag fees when checking in with AV and expect no credit card nor elite benefits.