ok, the more I think about this, the more I'm unsure you even want to do it this way.
Why not set the reminder email as a separate batch campaign? Then you can schedule them at precise times.
That way if they register after the batch campaigns, they won't receive anything more than an reg email.
You could also turn off the registration 1 hr before by deactivation or by stamping the date on the record and then say if Activity Between X and Y, then don't register the person and send them an email.
Also, why are you rejecting anyone?