I personally don't think it will kill them since they survive in that temperature when they are dormant, but it would sure make things really slow for them, but I would personally just keep the heat on all the time, no reason not to, the heat mats and stuff usually don't take a lot of energy, and the heat lamp if it is a ceramic one or something that is higher wattage, just switch to a few CFL lights or something or whatever you will be using and keep them a little closer to the soil, in a box or something like that.. will keep them warm, it will also help if you have all of them in the same enclosure, even if it's just a cardboard box for now, keeping the heat in will help
-and don't want to jack your thread but since it's about seed survival I figured I would throw this in here if you don't mind, but for anyone that is looking, if they know if the seeds will be killed if the substrate is allowed to totally dry out, for a day or 2, but the temps are kept the same, and it is watered after and back to "normal" for germination, or once the seeds start germinating, does drying them out mostly kill them?