Oh Jan, so sorry to hear that Spencer is having more seizure issues!
It sounds like you have a very capable vet, though. So that will help emensely. I know I have probably asked this already, but have you had his thyroid tested yet? (6-panel, sent off to HemoPet.org for interpretation) With Shelties being prone to thyroid issues even without epilepsy, and the connection between thyroid function and efficiency of anti-seizure medications, it really is worth the extra time and money to get it checked out.
As for feeling secure again ... it happens eventually. I usually go for about a month or two after Gio has a burst of seizure activity before I can sleep properly again. His recent seizures all occured at between 3-4am, that was a couple of weeks ago and I am still waking up at 4am on the nose to check on him. Pretty good internal clock!

Any little twitch, and I wake up to check, even if it is Romeo running in his sleep. I tend to also get night terrors and waking nightmares because of the stress and lack of sleep. As long as he doesn't have any more seizures, all that lasts about a month or two before I get to sleep through a night. But things quiet down eventually and you will be able to relax.
Here's to hoping that the PB in addition to the KBr helps. We've got our fingers crossed for you and Spencer!