I don’t know if a nasal flush would help. But a culture to identify the bacteria causing the snuffles is a very good idea. Snuffles can be caused by lots of different bacteria, not just pasteurella. She might need another antibiotic to get rid of the bacteria she has. Antibiotics kill off lots of bacteria that are sensitive to that type of antibiotics, and that leaves the field open for other bacteria that are not sensitive to that type of antibiotic. Then you have to try another kind.
Sometimes long courses of abs are required to kill off stubborn bacteria, bacteria are (sadly) very good at “hidin”g in small narrow spaces like the sinuses. They form a reservoir there and exist in a practically inert state that renders them insensitive to the antibiotic (most antibiotics targets bacteria that are actively multiplying). This may be another reason why your girl doesn’t seem to get well.
I think it’s great of you to take her in. It would be so sad if she were to be put down because of snuffles. You should be aware though that both pasteurella and bordetella can be transferred to piggies from a bunny. You should keep them in separate rooms.
She will have a beutiful life with you if you love her. If she gets really bad and declines physically, you can’t do much more for her. But chances are she gets better if she is in a loving home and gets good nutritious food and a nice living-space. All those things help boost the immune-defense, and if the immune-defense gets strong enough maybe it will be able to beat the snuffles. The immune-defense of young individuals isn’t fully developed.