Im sorry about the loss of your girl. A sad time of year with her anniversary approaching.
Also sending out some healing* (((vibes))) for Gulliver.
I think trust your vet on this. Head tilt is more often related to ear infections and given the vet can see it is plugged up, it makes it even more likely. Outer ear infections can move to middle or inner ear, causing some vestibular symptoms. Did the vet remove that plug as best possible?
*If* symptoms were caused by EC, the metacam he is on would help with any inflammation which is what brings on the symptoms.
EC is very often a presumptive diagnosis because it is difficult to get true results testing for it. So, often in cases of head tilt when ear infection isn’t obvious. rabbits are put on med to cover both possible ear infection and EC infection. Im not sure why the rescue gave such a dire warning.
You could give your rabbit a course of panacur after he’s finished on current meds if you wanted. I know in the UK, a lot of rabbit owners do routine treatment with panacur. I believe the product indicates to give a dose for 9 days, every 6 months. The negative side effects of panacur (fenbedazole) are linked with longer courses. The widely used 28 day course is thought to be enough to treat active infection but not so much that it will cause bone marrow suppression. Even so, some vets will do blood tests during this period to monitor for that. Individual rabbits may respond differently to the medication.
You could talk to your vet about adding in oral baytril to Gullivers treatment, if you wanted. That would help with any possible ear infection but it also can help suppress spore formation (EC, being a microsporia).