You might also want to consider insulating the darkroom to prevent the cold air (and keep it upstairs where it belongs). The easiest product would be reflectix or the stuff from I've used both. These are not messy like most insulation.

Your electric heaters might be allright, but I'd not trust my life to them underwater, even if you do have a GFCI outlet to plug it into. If a footwarmer gets compromised on the floor, the path to ground will be the floor not your body. One product I have used outdoors in wet conditions (melting groundfrost) is the powerblanket. It doesn't get super warm. It'd be great for keeping trays warm, but it's kinda expensive.