Max battery discharge current is about 150A. Not sure where you got that 141hp number, but that's just not the case.
Heaters that aren't forced-convection based aren't likely to be very effective as you're trying to heat an 82# insulated block of mostly nickel. If you installed some sort of forced air heating that pushes air in from the trunk and exits in the cabin, it would be double plus good as you'd heat the interior as well. This also increases the surface area you're heating by about 30X.
Think multi-speed/multi-heat
hair dryer (maybe not strong enough from an airflow perspective).
The thermistors are on the bottom of the pack, so they will be the first to be heated even if the pack itself is notably colder. I suspect it would need to run for quite a while to ensure the pack is truly above 50°F at which point the hybrid system is pretty much unrestricted.