Some of my fellow central Minnesota grower friends tried the winter seed sowing thing last year. After a long germination period The plants were weak to start, then took off, which was bad because we had like and extra month of winter that year, so they all died. I started mine indoors much like your setup, but just waited an extra month, and was about right. Once the seedlings are up and looking hardy, you can keep them cooler to slow the growth until they are ready to go out. Coldframes with old windows and strawbales work great.