I think the first order of business is do an experiment to see if the coolant loop can take an additional source of heat added to it. A proof of concept test before I spend the $.
My expectation is that I turn on the diesel coolant heater when the cars normal heater is turned on, both heating apparatus come on, warm up the coolant loop, then the diesel heater stays going and the 380v DC heater drops off and the diesel heater provides all the heat.
Over the years I created multiple 240v based hot water heater element based coolant heaters. I would plumb these into the ICE coolant system. I would start the vehicle to get the coolant flowing then energize the hot water heater element. A 5,500w hot water heater element added before the heater core would almost instantly start thawing ice off the windshield when plugged in.
The first ones were really crude just a 2,800w element and screw together galvanized pipe bits and brass hose barbs. The latest generation used welded 316 stainless steel tube, stainless hose barbs and a 5,500w folded over element. ThE first one was made back about 2006, the all stainless welded version started in 2010.
Since I know I have 1 or 2 of these heaters in my junk collection I would plumb 1 into the leafs coolant loop just to see what happens.
The 2011, 2012 and later sport leaf heater unit appears to contain the heating element and the temperature transducers together in the same unit. So I would want coolant to flow through the diesel fired heater or 240v AC electric test heater before it goes to the leafs existing battery powered heater.
Is going to get you.