Nothing is sacred anymore, not J nor F. Its all me, me, me or them, you get my drift.
But I certainly feel your angst, I saw one for myself PER to SYD.
One min a seat in J was empty, right across from me, I had paid $350 for that upgrade thing, $350 good worth it dollars, and no extra VA SC, mind you.
The next sec, this was after take off, this kid appears from Y and just takes that empty seat in J.
I was watching the empty seat from the start, its wasnt like the kid had boarded in J, but appeared from behind the curtain.
Not sure if it was sanctioned by a VA FA or did the kid daringly pop himself into J.
Then he started chatting with the lady in row H, by the window, so it could be deduced that maybe they knew each other.
For the thousands of extra that a person would have to pay LHR to SYD via DXB, or US to AU, to see a J or Y pax use a F loo, yes, kicking up a stink would be appropriate.
As for how OP was treated by the crew and now, by QF management, poor. But nowt much that can be done.