A lot of places will say that e.cuniculi or EC causes head tilt but normally what happens according to my vet is the EC weakens the rabbits immune system and allows infection to set in. Inner ear infection is the most common cause of head tilt. It can also be caused by trauma and cancer or neurological problems, but because your rabbit had ear mites I would guess the tilt is from an infection caused by the mites.
Keep an eye on the mite situation to make sure the treatment worked. Normally I've heard of it being given once a week for 3 weeks in a row. If it wasn't given every week I would think that may give the mites the opportunity to reproduce. Ivermectin, if that's what you where treating with, only kills adult mites. It does not kill mite eggs or nymphs. That's why you need to retreat as the existing eggs and nymphs mature into adults.
Normally mites or anything else from the outside would not be able to get into the inner ear because it's blocked off by the ear drum. If the ear drum ruptures I suppose maybe mites could get deeper into the ear, but I'm not sure.
The head doesn't tilt because it's heavy on one side, it tilts because swelling inside the ear effects the rabbits balance. Kind of like how when humans get ear infections they sometimes experience nausea, dizziness, and loss of balance.
I don't know anything about the APEX ear drops put make sure if you are putting drops inside the rabbit's ears that the ear drum is intact. Putting medications or anything else into the ear if the ear drum is ruptured can cause serious issues including death. Your vet should be able to look into the rabbits ear with an otoscope and tell you if the ear drum is intact or not.
You can do some research on rabbit safe antibiodics to make sure the antibiodics in the drops are safe. Some antibiodics are not safe to use on rabbits and there are others that are safe only if injected.
If you've looked and there really are no rabbit experienced vets in your area, would you vet be willing to at least do some research or consult with a rabbit vet?
Keep an eye on the bunny. Head tilt from infection can progress and cause some nasty symptoms such as a worse head tilt, eye twitching, rolling, loss of balance...etc.
I'm not trying to scare you, and I know an ear infection or ear mites doesn't sound all that serious, but it can be. One of my rabbits has been fighting an ear infection and head tilt and I'm afraid if he doesn't start responding more to medication soon that we may loose him
It's been a very stressful experience for me so far. The sooner you figure out whats going on and hopefully find a medication that works, the better the chance your rabbit will make it without any lasting effects.