Is there car older than 2004 as they made all cars come standard after that with isofix points just may not be marked up like they are nowadays?
Generally most do have the option for both but would depend on the car seat. It's also just as safe as isofix if fitted correctly. There's just more margin for error doing it manually with a belt xx
If you have a look on the car seat itself it will tell you. There's usually instructions on the back. Mine can and it shows you how to use both options. I would double check their car too though because most cars these days have the points as standard.
As long as your car seat has the adapters for the diagonal seat belt then yes. It's not considered as safe as using isofix but it is still allowed. I have the same scenario, my mums car doesn't have isofix so we use the seat belt. I only ever use that for short journeys though.