I once had a 2 day green laning adventure in the Welsh mountains in snow.
I had absolutely no problem keeping warm because of the amount of work I had to do to stay on the bike, but my goggles kept steaming up.
When you ride slower your breath has more effect upon what happens to the inside of your helmet.
So I smear a small amount washing up liquid onto the inside of my visor if I'm anticipating a slow ride.
Having said that, if you've got an expensive bike, it's not worth the risk of throwing it on the floor cause once it starts to go you'll do well to stop it.
Even at 5mph. So get a job in a building with a foyer & double doors with no steps up to it, oh...and a Redcare alarm system, so you can leave your baby there overnight if it snows.