evidently more common in older dogs, which doesn't rule out the occasional early onset, of course. It sounds like Molly's bleeding is very closely related to the seizures, though. I'd think that if it were cancer, it would be bleeding when Molly is not seizing, as well.
Certainly talk to your vet about your concerns and see what he suggests. Xrays are a good first step. He might decide to flush her nose and see if they can detect any abnormal cells in the wash. I hope they don't find any cancer cells!
Good luck, the girl's mom! Let us know how Molly is doing!