Nice work on the electrical plug install! I think that's a decent solution when one is willing to cut a hole in the bumper. Its a bit too extreme for me, I have always preferred to find a way to mitigate the dangling cord. I have one really nice, super clean solution I had done on the fiance's previous car but we no longer have that and I'm not sure I even took any pictures of it ever. Guess I'll have to revisit it, been meaning to look into getting onboard chargers for our cars.
What I've found is that the #1 thing to guarantee an engine start in extreme cold of northern MN is keeping the battery voltage up, especially in these newer vehicles that have radio and security electronics that are always using a bit of juice. You can heat an engine block up to 175 degrees with a zerostart 1500 watt coolant heater and thats awesome for being nice to an engine in the cold and for getting warm air in the cabin right away, but if you mainly just want to ensure it will start, period, the most critical thing is just for the battery to be charged. To that end, I've somewhat switched away from doing block heaters and coolant heaters and nowadays I just go for an onboard automatic battery charger/maintainer. The charger makes sure the battery has maximum power to crank the cold motor over, and the trickle charge current prevents the battery from freezing (the colder batteries get, the less output they can produce).
It may not be kind to crank over a cold engine at -65 degrees but you can do it and it will start and go if the battery is at 100% and not frozen. Obviously more wear occurs when a very cold engine starts but its debatable as to how much impact it will really have in the overall longevity of the motor. My contention is that as long as our cars have fresh, full synthetic oil going into the cold part of winter then the handful of extreme cold days we force the cars to start will not incur enough wear to noticeably impact the number of miles we will get out of our engines. As added bonuses, I don't have the cost and complexity of coolant/block/oil heaters, no noticeable impact on the electrical bill to run a ~1 amp charger, and our alternators are happier and live longer.
So take that 2 cents for what you will, to anyone who lives in cold climates if winter sneaks up on you and you are sitting there thinking "dang, I should have gotten a block heater like mudman, now its supposed to hit minus 50 tonight!" If you have an automatic/trickle battery charger/maintainer in your garage (they are cheap), you can at least run out and hook that up quick and then you can pretty much guarantee the buggy will take off in the morning. It may be unhappy about it (it may be SUPER unhappy about it if its an old 10w-30 oil vehicle loaded with conventional oil) but it will probably go as long as you make sure the battery has the juice to crank it over.
If you are trying to get 300,000 miles out of your motor without a rebuild and it hurts your heart to listen to the cold motor clatter when it fires up, then definitely go with both a block or coolant heater AND a charger/maintainer