Even though Qantas is acting purely as agent I would expect that Qantas has arrangements with its partners to deal with these situations ie. if Qantas steps in to provide compensation it should be to restore the customer to the same or better position they were in at the time of booking.
Qantas can then recover from JL as the situation arose from JL's decision.
What makes this situation worse is that it's not like a factor out of JL's control caused the downgrade eg. weather. They made this aircraft type change as a commercial decision ie they wanted to put this aircraft type on a more profitable route.
So in that case they should wear the "costs" of making this change.
Whether a customer is paying cash or points and flying F, if they book F and at the time J or Y was available, that is the customer signalling that they perceive better value in F and lesser value in those other cabins.
To put them in those other cabins and still charge them as if they booked that cabin in the first place doesn't compensate the customer or restore them to a similar or better position.
Something like 150% of the fare difference back would be more reasonable.
I'm surprised at the reaction.
If we don't put pressure on airline companies to increase or better their compensation offers we'll always be stuck with poor offers.
