Just how do BA expect QR to fly pax to JED, RUH, DMM and BAH?

so what is the answer when someone is in say SEZ on a reward booking and BA have cancelled all flights back for a month?

You sit on your tropical beach until BA runs a flight, then send BA the hefty bill when you get back.

Something I have been considering.

With regards to EU261 BA will claim
they don’t need to compensate due to extraordinary circumstances. However, they have beeen flying all over for example to São Paulo, Nairobi (on the day we returned), etc. Yet they cancelled select flights due to commercial reasons. Surely this wouldn’t stand up in court as due to COVID if they were still
flying in and out of the country to other destinations?

There is some commentary, advice, and a report of a CEDR adjudication around this subject on the EC261 thread. Basically BA are very likely to reject any claims atm where compensation could be due with under 14 days notice of a cancellation. You would have to take it at least to CEDR/MCOL and argue on the two points, that the cancellation was not caused by extraordinary circumstances and it could have been avoided even if all reasonable measures had been taken.