The GA can (re-)assign seats (if need be, in a higher cabin), hand out vouchers or at least see to that the PNR gets annotated about the involuntary seat downgrade so the OP would have had a way easier time to claim the refund of the seating fees due. Regular cabin crew can't do any of this, not on QR. Probably the CSD (Purser) can, but apparently the OP didn't even insist on getting them involved - which would be the first thing I`d have told the FA when faced with someone sitting in my seat.
And trust me, faced with the task of getting the flight out on time this usually gets resolved to everyone's satisfaction.
Traveling with a service dog and having to experience that a substantial demographic of QR's customer base isn't very receptive to having a dog in their immediate surroundings, I`ve had my share of involuntary seat-reassignments resp. attempts to do so on QR.

Fact is QR treats the hole upper deck economy cabin as "preferred seats" and charges non-status passengers or those on discounted fares a considerable amount of money in ancillary fees to sit there: https://www.qatarairways.com/tradepo...-Purchase.html

There's airlines who will state in their CoC that seating fees are nonrefundable. Qatar is not a member of this club: (click):
You shall be entitled to a refund of your purchase in the following limited cases:

• (a) You were not able to use the assigned seat for reasons attributable to Qatar Airways including but not limited to safety, security or operational reasons, overbooking, change of aircraft, seat re-assignment and the product you originally purchased is not available but excluding reasons attributable to your action