I'm "at home" with independent diesel heaters as I've used some for ages.
I would share some recommendations.
First of all, 5kW heater is too powerful for use during driving.
Looking at cheap Chinese masterpieces those 2kW versions should be just the right size.
Heater itself can be installed wherever possible but inside the cabin is not the best choice.
If we talk about air heater (2013+ Leafs) then I would recommend ducting warm air outlet
next to driver footwell. Also rear side of center console seems fine. Idea is to have fresh
air close to front of the cabin as the exhaust is in the trunk. This way heat will do more work.
Cold air intake should be external. This will slowly defog windows as well.
Burning chamber exhaust should be routed away from cold air intakes (cabin and burning intake)
and also not close to front wipers. It's fine to leave exhaust blowing under the hood or at the ground.
Leave tank under the hood and "in the reach of the fuel station refill" so direct refill (with accidental drips) is acceptable.
Close to windshield washer refill is a good place.
Refill twice a month is a good start. Tank size according to daily commute:
1 hour of driving a day, therefore 14 hours per 2 weeks, therefore 3 liters.
2 hours of driving a day, 6 liters. I don't recommend going over 7-8 liters. OEM tank might be perfect.
Those who think about environment: Diesel heaters are very efficient if we measure well-to-wheels efficiency.
Using solely PTC/resistance heater that many Leafs have, is worse than burning diesel (average grid carbon footprint).
Removing the heater for summer is acceptable, but keep the ducts alone, those have no weight
2011-2012 Leafs have liquid core for cabin heating. Those Leafs can accommodate a coolant heaters as well, but
adjusting those is more complicated. Usually too powerful (4-5kW is too much, 1/2 power is also a lot).
In case of adjustable independent heater power: do not turn it too low. Keep it above 1/2 of max.
Burning is not clean if fan speed and chamber temperature are low. Requires purging @ full power.
Heater fumes are not dangerous if accidentally inhaled. Mostly CO2 and water vapor.
Heater consumes 1/5-1/2 of what diesel car consumes at idle.
It's OK to use independant heater for 10-60 minutes before departure. Good 12V battery will handle that with ease.
If vehicle is charging/preconditioning during preheat 12V battery will be constantly fully charged.
Sketching the ducting for warm cabin line (consider preferring driver side footwell or even 50/50 split with some additional
piping and one additional outlet, should cost few bucks) :