No reason why a diesel heater shouldn't work well. My diesel has one, slightly larger than a house brick and 5Kw output and warms my car up quickly, whilst still sat on the drive, if I turn it on. Other than turning it on, it kicks in automatically if I start the car and the outdoor ambient is below 10C and the coolant is below 75C, to speed up warm up once on the road.
The point is more why have a diesel heater if whole idea of electric car is not to use diesel? I can see the point in a very small battery to gain energy when slowing down to use it again when speeding up, I can also see the problems with a mechanical drive train when pulling heavy weights, the diesel/electric used in many railway engines makes sense. I can also see the point in a small liquid fuel engine to extend the range of an electric vehicle, same with solar panel on the roof. I can also see the point to be able to switch to electric in congested cities and tunnels.
Both my wife and I have ebikes, hers is mid engine and drives through the gears, mine is built into the rear wheel, they are very different to each other, on the flat both are equal, but on inclines mine if it can't maintain 6 MPH starts to loose torque, so I reach a point where I can't go fast enough to balance well before hers. So I have to get off and walk before her.
The same applies to many electric vehicles, so quirk with one make means it does not preform anywhere near what the market leaders do. For example we read about 22 kW charge points, but few point out many cars can only use 7 kW even if the point can deliver 22 kW, range is another point, for a van used at low speed delivering milk on the Wirral, with heaters off, when the range is claimed at 100 miles, should not have a problem with a less than 70 mile round. But it did.
I looked at fuel consumption of a Vauxhaul, they worked out how far it would go on one litre of fuel starting with fully charged battery and ending with fully discharged battery. Then times the figure by 4.54 to get miles per gallon, but the battery is not 4.54 larger, so it could have never done that many miles on a gallon.
If you took the range on a tank full, and divided that by gallons in the tank, one came up with a very different figure. Which was not as good as the non electric version, so why bother with the electric?
The whole reason for using electric is the way it is taxed or not taxed by the government, and the way the licensing laws work, it has nothing to do with saving the planet. My ebike is really the same as a moped, slightly slower than my old P50 Honda due to law, but around same power and same range. But moped not allowed on tow paths, or cycle paths, and I need a licence, I got ebike when I became 70 and it took from Jan to Oct for the DVLA to renew my licence. It ran out in March.
The whole ebike thing is also silly, if built before a date it can have a throttle, but not after that date, but unlike a car, there are no documents saying when built, so my second hand ebike has a throttle, and even I don't know if legal as no idea of how old it is.
We have the same with EV charging points, regulations have been made to ensure they are safe, but the regulations are not a legal requirement, so touching an EV on charge could be lethal, we simply don't know, unlikely any commercial charge points have an issue, but domestic is another story.