Seizures are tough because there are so many causes. One of the hard things for vets is that they can't ask the patient about symptoms; they can only go with what they can observe -- which can be extremely difficult if the animal isn't actively seizing at that time.
The lump seems as if it would be easier to diagnose. However, once again, without the patient being able to answer questions, misdiagnosis is possible. My vet of more than 20 years diagnosed my cat as having a bad ear infection, based on symptoms. However, Pumpkin actually had a cancer deep inside his ear.
Once we figure that out, I then had to decide whether to treat Punks with medication and hope for the best or let the vet perform surgery to try to remove the cancer. I opted for surgery, which in Punks' case, bought him some quality time -- until it didn't. Even with the best treatment, just like with people, there is no guarantee. However, I made sure Punks had the best life possible for as long as possible.
Having to make life-or-death decisions for another living being is so hard. I hope your vet figures out what's going on with Joey, and that you and Joey have more good years together. Please let us know what's happening with him.