Thanks for the ideas. The 2015 model battery charges not with necessarily with long runs, but with periods of deceleration due to the "smart" ECM management for fuel economy, or during night loads.
On the night in question, the battery was likely at about 12.25 volts, perhaps just marginal enough not to crack the relays if "stuck" or frozen. In addition I now found out that the plugin source breaker was turned off -- so the engine did not have the heated oil advantage. The 0W20 flows relatively well regardless.
Nevertheless, the key turn at first did not engage the starter at all. When it finally did, the engine turned over well enough. So in this case I think I can rule out a faulty heater block cord.
How am I addressing this? For the battery, I read and confirmed that a full charge can be force during daylight if I put on load like parking lights. I may also get the battery TSB done (trip to the city and $75). I also gave the battery terminals a slight wiggle and extra tighten.
I also confirmed that the relays are indeed the new ones (eg. M09) as specified in the relay update.