Marigolds generally sprout very quickly, so if it were me, I'd go ahead and sow more. Are you sowing fresh seed? Some varieties don't keep well. Although mine haven't done well this year either and I bought new seed this year. Could your oven (never even thought of doing that!) be a bit TOO warm?
No idea on the eggplant seedlings, tho. I stopped planting them when I got jabbed in the finger trying to pluck a fruit!
