And another (amazing) update: we went back to my bro-in-law's house for Dec 24th/25th, where my dogs first picked up the flea infestation that started Jimmy's allergic reaction. (They have TONS of squirrels, opossums, raccoons, etc that live in their neighborhood)
My dogs would go out into their backyard to potty - and come back into the house. Within a few minutes - DEAD FLEAS would surface on their fur!! I was stunned that the Bravecto worked that fast!
Unfortunately, my suspicions were correct, though: in order for the Bravecto to kill a flea, it has to bite its victim. So Jimmy still has multiple itchy welts from the bites - but he doesn't have the angry-hives reaction he had from my first post. He still scratches a lot - but isn't chewing/tearing at his skin like he did before. He's back on Benadryl at night, to make things more peaceful for ALL of us.
Ben, who doesn't seem to have flea allergies - is fine. He spent two days gathering fleas, and didn't bring any home to our house ;-)