Coincidentally my own personal cat who has chronic nasal discharge and sinus infections from herpes virus (which may also be to blame for this little guy's problems) just got out of my hospital following a head tilt and imbalance from ear problems.
The signs you are seeing may be related to the ear cleaning itself, to a deep inner ear infection
, or to toxicity from the ear mite medication if his ear drum was not completely intact due to the mite infestation.
His fever may mean he has severe inflammation, an infection, or both. The penicillin you have may not be adequate to treat a deep ear infection as usually these infections require antibiotics in a different spectrum (ie. to treat gram negative infections if you are familiar with some microbiology terms). For inner ear infections usually 6-8 weeks of antibiotics are needed because the infection is considered a "bone infection" and antibiotics have a hard time penetrating bone.
When a cat develops vestibular signs (imbalance and a head tilt) steroids are usually needed to help reduce inflammation of the nerve that is being affected.
The good news is his signs will most likely begin to resolve in 3-5 days with treatment. He may continue to have a bit of a residual head tilt but could go on to be normal. In addition the long antibiotic and steroid course could help with his sinuses and maybe help prevent him from having to be put to sleep.
The bad news is the signs may take as long as 3 weeks to resolve especially if they were brought on by the medication reaching the middle or inner ear because of a diseased ear drum. (Mite medication is safe normally but can severely damage the delicate cells and tissue in the middle ear.)
I hope this is helpful and I feel for you and your kitty as I know how hard it is to watch them struggle.