You mentioned having her tested for hyperthyroidism but did you have a complete blood profile performed or just this test?
How old is she?
Does she have increases in thirst or urination?
The attacks sound just like seizures but this doesn't normally come with weakness in the rear end. I know you don't like to take her back to the vet but it is time to get a referral to a neurologist. I for one would be sure that she has had a toxoplasmosis test run and other infectious disease panel. Remember that it is possible to have the two conditions be completely unrelated. She may have hurt her back or her spine in the last seizure. I would consider treating the symptoms while looking for answers. Phenobarbital is commonly used to treat seizures in cats and metacam can be used to treat inflammation which may be what is going on in her rear legs. A neurologist may be able to help you isolate the cause more effeciently then a nonspecialist at this point. My concern here is that these seizures are causing a mild brain damage and with each one it gets worse resulting in the other symptoms you are seeing.