I don't think there is a person here who isn't happy that Polo is feeling better. We are all just concerned that this is temporary and could recur at any time.
It could be vestibular disease (vertigo). There are different types though. The neurologist can sometimes guess at which type by the symptoms, particularly the nystagmus. Since the neurologist thought it was likely encephalitis and since they really wanted to do an MRI, central vestibular disease could be what they are concerned about. Idiopathic (or peripheral) generally goes away within three weeks without treatment. But the report from the neurologist indicates that she doesn't think this is idiopathic vestibular disease.
The fact that his symptoms got so severe (including not being able to urinate for days) with the fact that the neurologist is very concerned makes central vestibular disease more likely. This is just a set of symptoms that is probably part of an underlying problem. Steroids can help if it is something along the lines of GME; however, it is very likely that they will become ineffective (because without a diagnosis, the dosing may not be right).
If an MRI is out of the question and a neurologist does suspect either IVD or CVD, steroids and antibiotics can be started. Hopefully by doing this it narrows things down. It can cover the bases if there is an infection present while reducing inflammation. If symptoms start coming back, that is more information for the vet to go on. But again because the neurologist feels that this is a serious case, it seems that a diagnosis really is needed now to try and treat as accurately as possible. At the least, the neurologist should be making any decisions regarding what drugs and supplements are given. This is way beyond anything a holistic vet should attempt to try and manage. And with two vets giving drugs and supplements, there are two protocols being followed which could lead to more problems.
I don't know if there are any vets posting on this thread. My guess is no, but I don't want to assume that. There are, however, many Yorkie owners who do care about what is happening to your pup --- people who have owned Yorkies for many years and have seen countless vets and spend their time learning about the things that can go wrong with this breed. There happens to be a former vet tech or two posting on your thread too - one of which I know has worked closely with specialty vets. I'm not a vet or vet tech. I do spend a lot of time watching vets practice though. And in this case none of them would be sending Polo to a holistic vet. They would want him in the hands of a neurologist where he could have the best chance for a diagnosis. And when the specialist (neurologist) sets up a protocol, they get the support of the animal's regular vet and the owner isn't diverted into starting a second protocol.
Please make sure to check often for UTIs. If he is having trouble urinating, infection is common. If at all possible, an MRI to rule out some things would be in his best interest.
http://neuro.vetmed.ufl.edu/neuro/vestibular/vestib.htm