I will say, i don’t think that in your example, with the engine running every now and then to sustain heat, rather than just driving the last 3-4 miles on the engine itself makes a big difference in consumption. Particularly if we take into account that, if the engine in your car is running for the heating at a red traffic light, it will also use it to accelerate “while it’s at it”.
As described previously on how the previous medel provides heat and indeed mine but, untill I know any other we’ll assume they work on the same principle.
If heat is demanded over a setpoint a couple degrees over ambiant, the engine turns on and maintains a high idle, say 1050rpm. Coolant is then sent through a bypass valve at the exhaust manifold, quickly heating the coolant to reach the desired cabin temperature, at which point the engine turns off, untill such a time the process is needed again to maintain the set temperature or the user demands a higher cabin temperature. Predominantly the car is still maintaining electric running, provided there is enough juice in the battery for it to do so or the car will switch to hybrid mode if not.