The tumor is not likely to have moved, so I suspect that if you take her off the drugs, the seizures will return.
It does sound like she might need some adjustment of the dose of phenobarb, however. Is there no other vet at the clinic that might be able to offer some advice? I wouldn't try to adjust the dose on your own without some guidance. However, the vet might be able to reduce the dose of phenobarb to a level just strong to control the seizures and prescribe something else for any pain your girl might be feeling.
Good luck with your girl, Sad about dog.