OP:
It seems to me you had a lot of expectations, which you formed based on what YOU think should happen
not what the airline actually offers regarding service animals.
And if on the respective webpage (
https://www.aa.com/i18n/travel-info/...ce-animals.jsp) they don't mention MCE or priority boarding, then I would have assumed that in fact nothing was going to be offered (whether it makes sense to you or not).
About the check-in, I don't know what's required but it makes sense to me that they require in-person check-in. What if you say you have a Golden Retriever and you have a dragon instead? That the dog you filed papers for is the dog you are actually traveling with probably needs to be checked at some point, not last minute as your boarding pass is scanned at the gate.
I can just smile at the comment of the friendly/unfriendly FAs.
I am glad the first group of FAs treated your dog better than some FAs treat humans.
What you wrote could be perfectly applied to a passenger. See below. It happens.
4. On-Board: ...On the way home, the flight attendants seemed more hostile about it. As we boarded, one flight attendant seemed to have been waiting for us, wanted to confirm our seat assignment as we boarded, and was heard making negative comments about US service dogs as we made our way to our seats. In neither case did WE the service dog cause any disruption or issue at all (I'm pretty sure almost no one around us even knew WE WERE she was there), but it was interesting to see that even if you do everything right, the response will vary, not just from other travelers, but also from the crew.