The parasites I mentioned are ones that migrate to the brain. Toxoplasma, Ascarids (roundworm larva) and Neospora are examples of common parasites that can encyst in the brain. The parasites could have been there for years (since puppyhood) or from a more recent infection. They are widely distributed in places animals live including woods, sidewalks and parks. They are usually picked up from infected feces or places where infected feces have been (sidewalks). Parasites can cause seizures. That does not mean that they caused your pupper's seizures.
If the softball sized mass was food, it would be not be there if another image was done. Usually, a cup of food has moved out of the stomach in well under 4 hours. However, a liver mass should not be causing seizures because it does not affect enough of the liver to interfere with the normal function. Seizures from liver damage require the liver to be severely affected so that it is not able to function.
Seizures have preictal and postictal phases. Vomiting, diarrhea and urination can occur in any of the phases including during the seizure.
Seizures always originate in the brain. The cause must be something that affects brain function. Masses (tumor, encysted parasites), inflammation (encephalitis), lack of needed nutrients (glucose, calcium, oxygen) are major known causes.
Seizures by themselves do not cause weight loss. If they interfere with eating, they can.
Have you and your vet treated the seizures with anything yet?