Wow, sorry you're having to deal with this man.

Well it sounds like the central nervous system.
The causes can be Trauma, Chemical, Viral, Parasitic, or Organic.

If you are sure that there was no trauma or chemicals, then with organic, do you really want to spend hundreds on diagnostics and treatment? Same with Viral and parasitic. But if you think it may be one of those, (v/p) then yes, quarantine the animal.

I understand your reluctantance to perform a CD. You can always use a shovel as a guillotine. Labs use an expensive guillotine.

Depending upon were you live, a .22 bullet at the base of the skull is instantaneous. I've done it successfully with pets over the years with convulsions. You think it may be hard to do for a rat, think about a beloved 10 year old dog!

Best