Re: QANTAS- Not to be trusted!
As Medhead suggested, request the Original Routing Credit for your QF booking. When you receive this, you will be pleasantly surprised.
Also there should be a few other things coming your way from QF as a result of being caught up in the disruptions, that all of us in that situation received. Have you received any of these?
BA should be interested to hear about your on board issues, where things should have worked. You would have notified the FA on board about these especially seeing it was so many things, so they would have a record of it. You no doubt asked for a different seat as well. I'd be approaching BA because you were their customer, not Qantas'.
All other carriers would have been scrambling to pick up the extra load that couple of days. And QF would have been dealing with thousands of extra calls & requests, yours & mine included. If I had more than the usual problems with UA I would be expecting them to make pretty with me, not QF.
Unfortunately from what you have posted, QF offered to clean up BA's mess, whereas in a normal situation they would have pointed you in the direction of other avenues available for the responsibility for inflight issues. Better IMO to have a can do answer, than having "no" as the default answer in those first few wild days, which is what seems to happen with many o/s carriers. Stonewalling, refusal to even discuss, put it in writing Sir, go to the website for the complaints department, click.....Would you have been happier if they had said no first up?
I can understand that you thought you had an answer, & were surprised by their offer & looking forward to a part refund! which is something I have never experienced for inflight issues, only to find that it has changed. In my experience, if something sounds like an unusually good unexpected bonus, it's probably a mistake. If it ends up not happening then I cope. Liars is a bit strong. And thanks for nothing? What about all the other things they are doing? Return fare, voucher, original routing credits ....
Please let us know what BA end up doing, and how they handle your complaint, as well as what you end up with from QF.