You cannot have more than one brake valve controlling the brake pipe (except for Locotrol remotes on today's diesels). Some railroads used to allow a second brake valve cut in to assist a STANDING train to recharge the brake system.
Cold weather continues to cause problems, mainly due to increased leakage and the possibility of condensate freezing in the brake pipe (usually in the gladhands) and restricting or blocking the brake pipe. I imagine the extra cooling of the warm compressed air further drops the pressure. The use of alcohol had been prohibited (brake pipe and otherwise), it destroys the gaskets.
The M-3 would be a more "modern" feed valve if used with 6-ET, more common would be a C-6 (of earlier design and less capacity).
I've railroaded several winters in below zero - it is not fun.