Don't know anything. But before I ask I research previous threads. usually the question has been asked and answered indept. There are numerous threads on germing seeds.
I'm a soil guy like Lo Budget and his method sounds very close to exactly what I do. just started some kandy kush that's about 3yrs old and they're doing great. Have you tried starting them upstairs first then going to the basement? And just my opinion Lose the heat mat.
i was thinking filter > fan > flow hood > duct out other room the intake to the room would be negative pressure and the only airway would be the cracks in door and sealing
if air can get in through cracks and ceiling then unwanted light can too. also during periods that fans aren't on...