The battery has a great deal of thermal mass so it's not going to heat up or cool off quickly. One of these days I will be curious enough to see what my battery temp is after a remote start, when I get to work, and after sitting all day at work in a typical 20ish F northern Ohio winter day. I have measured the current going into the battery on regen. I don't remember the exact numbers, my very fuzzy memory makes me want to say it was about 50kW (160A x 325V) going into the battery under max regen. I could be way off here. I don't know at what temperature cold charging becomes a bad thing but everything I've read seems to indicate it's somewhere below freezing and gets exponentially worse the lower temp you try to charge at. So if your garage is a balmy 40F and you run multiple short errands during the day, I wouldn't bother plugging in until the end of the day unless you'll need additional range. There really wouldn't be any reason to precondition either. As far as preconditioning heating hte battery, I would think 900W from the battery heaters alone would take some time to appreciably raise battery temperature. However Ford doesn't offer much info on how the 3 coolant loops interact, so without doing some testing it's hard to say how much preconditioning heats up the battery. Who knows, maybe heat from the cabin heater and waste heat from the charger are used to also heat up the battery... I think you've peaked my curiosity enough I'm gonna hook up my laptop and do some temperature readings this weekend.
I could be wrong but I thought it was only 5% on the bottom end. The 80/20 rule sounds about right to me. I try to do 80/30 when it's warm out. A little higher on the low end to give me a few more reserve miles. I do 70/40 with my model 3 because the battery is so freaking huge. The thing that sucks about the model 3 is the vampire drain as the battery TMS never shuts off. Anyway back to the FFE. If you want to dive deep into the battery specifics Idaho National Labs did testing on a 2013 FFE: https://avt.inl.gov/vehicle-button/2013-ford-focus