As long as the soil reaches the ideal temperature the seed will germinate.... Again it makes no difference if heat source is below or above the flat.

I've always used the heat from the lights before germination and never use bottom heat. My flats are in an unheated room, too. I've had very good success with my method and I wouldn't change a thing. Your setup sounds fantastic. My only question would be the flat in darkness, not under lights. If you need a heat source for this one, then use the top of your fridge. I still put seeds needing darness under lights before germination, and have put pieces of cardboard/kraft paper over top of the cell packs on these ones. The nice thing about being a beginner is that you will use tidbits from other people and find out what works best for you :)