You can give bromhexine to rabbits, it’s a decongestant. It’s sold as Bisolvon but has other brand-names as well. You use the liquid kind for babies/children. You can ask your vet about proper dosage. It’s OTC. You can also ask about giving her Benadryl, we have members who have used benadryl for babies on bunnies, it has some decongesting effect on the mucosa
I don’t know if nasal spray is suitable for bunnies. saline is of course harmless, it’s the spraying I feel a bit doubtful about.
You can run up a good steam in your bathroom and sit with her there for 15-20 minutes or so to help with congestion, but sit with her all the time so you see she doesn’t get too hot, buns are very sensitive to heat.
As for tummy health, there are some probiotics that have been shown to be beneficial for bunnies f ex saccaromyces cerevisiae (brewer’s yeast) and enterococcus faecium. Apart from that, lots of hay is of course the best thing.