Hey fellow migraine sufferer! I think the rebound headache should only last a few days at most.
As for the Tylenol PM, it has antihistamine in it, which I know can also cause rebound sinus pressure, but also only for a few days I think (in my experience).
I think the Topamax takes a while to kick in, I tried to take that to prevent migraines, I think they said up to a month of being on it.
If that doesn't work for you, I've had some success with being on a low dose of one of the tricyclic antidepressants, for me, 25 mg of nortriptylin (I know I spelled that wrong) at night not only helps me sleep!! (makes you a little drowsy) but has helped w/ the migraines a little, and sciatica a lot. And nortryp is way cheaper (generic) if you have to pay for it. I've also tried beta blocker to prevent migraine but that hasn't worked so well, but I know that helps a lot of people. Another one is neurontin, also helps with neuropathic pain if that is your issue, and will totally knock you out at night.
I'm sure your doc warned you that topamax has a lot of side effects, I'm really sensitive and couldn't stay on it. I hope that you are able to take it and that it helps you. As for the headaches now, hang in there. Headaches and migraines are horrid.