With a clear EEG and ct, is an epilepsy disorder still possible?

Yes. Eeg usually detects best coming from cortex an outer layer of brain. If seizure discharges come from deeper area of brain, it may not detect. Also some seizure discharges are intermittent. Ct scan is to find possible cause of a diagnosed epilepsy not the other way around.
Yes. If the eeg was normal during a spell, it argues very strongly against epilepsy. Otherwise, a negative eeg and ct are certainly possible even in patients known to have epileptic seizures. Sometimes multiple eegs are performed on different days in hopes of catching an abnormality.