I’m guessing your vet wants to xray her skull to rule out an otitis or a traumatism, which can both give that kind of symptoms. It’s normally done without any anesthesia so it shouldn’t be a problem – it’s quick and you can ask to come and hold her with the vet (I did it for Aki when she had x rays done back in December and she was surprisingly calm). After that, they can do a scan or an MRI which are under anesthesia. Some analysis to rule out parasits other than e cunicuili or bacterias or often done too in that kind of situation.
Of course, e cuniculi is an option too – it’s a pain there is no definitive test to make sure of its presence in the rabbit’s organism. I researched treatments recently because one of my rabbits suddenly became blind and I suspected e cuniculi. I found that Panacur is sometimes associated with Toltrazuril or Ponazuril with good results. You probably know you have to bleach everything to get rid of the spores. But from what I’ve read, the Panacur should be given for AT LEAST 4 weeks so the 28 days treatment is just the one that is initially given to every rabbit suspected from being infected. Discuss it with your vet, but you can continue with it for longer if the problem is not settled… the difficulty here being knowing for sure that it is an e cuniculi related issue…
If it’s e cuniculi, the damage done probably can’t be repaired – she will still lean on the side even if the parasit is not active anymore.
I understand why you are reluctant to put your rabbit through the stress of the tests. 10 years old is not young for a bunny… I guess it depends about her general state and if you feel it’s worth it to put her through tests and treatments (if you feel she can stand them too)