DFW has been one of my regular F trips the past 12 months and I can understand the desire to fly F instead of J on the A380 (although an upper deck window seat with the side bin is probably one of the best iterations of "old" CW).
I think trying to change a ticket at the airport is always going to be tricky and wonder if you might have had more success just asking for an AUP at the start?
But as you know the F cabin on that route (and really on a lot of them now that there is so much less F with the retirement of the 744s) is usually full.
The people who might have been able to help were the GGL line so a shame if you will now give up that privilege.
You are already GfL so you will be reasonably well taken care of even if you do only fly BA occasionally now and the benefits on AA are good too.

I have dealt with the station manager (or whatever the equivalent is now) at DFW on a few occasions and found her to be pretty helpful.
She knows my face now and will find me to pre-board which is a little bonus I quite enjoy.
I feel sorry she couldn't sort you out but I can imagine with everything so busy and the flight so full that it just wasn't a high priority when you already had a confirmed J seat.

There aren't a huge number of other options for that route if you are wanting to stay in F unless you want to stay with OW and use AA.
Did you use Flagship First Dining beforehand? A gem of the network in my humble opinion.
I have never come across the situation when there were no showers available in the arrivals lounge so I would chalk that one up to very bad luck.