Isn't there a provision for the traction battery to charge the lead acid?
Sorry if I'm a bit behind in reviewing the owner's manual.
As far as the brake lights during regen; there must be some threshold. My wife on the phone in a car behind me said they never came on as I forced a good bit of regen braking approaching a slower truck in smart cruise. YMMV
Yes - there are (slightly different) mechanisms in the PHEV and the HEV for charging the 12V battery from the traction battery. But I don't think this is a good option for preheating the car. A few years ago, I was going to a pot-luck dinner and I looked into the idea of plugging a crockpot into an inverter plugged into my cigarette lighter so that I could keep my contribution warm on the way to the dinner. After all, the crockpot only draws about 150 watts (less than an old 3-way incandescent lightbulb), which is way less than an electric space heater (typically those run around 1500 watts). But the cigarette lighter was rated for a max of 10 amps at 12 volts: e.g. 120 watts, and that's before any parasitic losses from running the inverter to convert DC into AC. I've seen what can happen to a wiring harness if you overload the cigarette lighter circuit and you're unfortunate enough to not have a fuse blow: it isn't pretty, and it's very expensive.
As for the brake lights, this is a really interesting question that I've been wondering about too. If you've had someone following you and telling you via phone when your brake lights come on, my bet is that you know more about the Niro behavior in this regard than most anyone else here.
I recall reading about a lawsuit many years ago where someone was going up a hill, took their foot off the accelerator to slow down and avoid cross traffic, and the guy behind them plowed into their rear end and then sued because they slowed down without ever signaling. I don't recall who won the suit, almost think it was the guy who hit them. But whether he won or not, just the fact that you could even be sued over something like this is food for thought.
When driving a conventional car, we can slow down by taking our foot off of the accelerator and also by shifting into a lower gear and using engine braking. Neither one of those actions will turn on the brake lights. When driving a car with regenerative braking, what we're really doing is something very similar to engine braking, in that the electric motor which normally propels the car is being used to slow the car down by generating electricity to recharge the battery. Until the brake pads actually make contact with the rotors/drums, there might not be an actual legal requirement for the car to turn on the brake lights: at least, the manufacturer might not be required to make that happen, but we as drivers might be required to signal to traffic behind us, regardless (and any such requirement might vary from the laws of one state/country to another).
However, there is at least one electrical switch that is triggered by moving the brake pedal, and in most cars, that switch does at least two things, even at the lightest touch of the pedal: it turns on the brake light and it disengages the cruise control. In the Niro, I believe that there is a second switch/mechanism that first switches on regenerative braking and then eventually engages the hydraulic brakes if you push the pedal far enough, hard enough. So I'd be very surprised if putting my foot lightly on the brake pedal was not turning on the brake lights at the same time that it was disengaging cruise control and engaging regen braking.
But when driving with smart cruise control where the car is actively slowing down by engaging regenerative braking, I've often wondered if the brake lights come on or not.. Perhaps they aren't legally required to from the manufacturer's perspective, because it's technically just "engine braking".