Marbles. That's a really great idea! I'm going to keep that one in mind.
Sadly, my JAO failed because of that same predicament; making JAO in a MN winter isn't a good idea.
Be careful to use marbles from a pet store -- ordinary hobby or game marbles may use chemicals to color the glass that can leach into the mead.
Food grade CO2 -- try a forum search but I'm pretty sure the difference is a gas that's free from oil or other contaminants that can come from the processing. Otherwise, CO2 is CO2