I finally settled on 1 hour off, 15 min on for the exhaust timer. The smell was fairly strong this morning.
I like the idea of hardwiring the carbon filter+exhaust fan so that it is constantly supplied with a minimal amount of current to drive it at a very low speed (maybe 50 cfm or better yet 1/3-1/4 speed it would take to empty the room in one hour), during the day cycle when it would typically be off. When the exhaust fan is supplied power (nightime) it would then act like normal.
The CO2 controller can make up for the lost CO2 easy enough, which should hopefully be minimal.
This would be a way to prevent any smells from leaking out, and hopefully help the other issues.
Anyway... just ideas at this point. Solve one problem and create two more!