I figure I'd throw my hat in here as well...
I remember the 2011 heater system. Hilariously complex, but efficient.
It consists of a completely separate coolant loop, but the "coolant" is rather used as "heatant" instead here. I think they just jimmied a standard Nissan HVAC system in there, one typically used for ICE engine cars.
In that coolant loop, there's:
- its own reservoir
- a heating element
- its own pump
- a heater core (goes to the vents) - and pretty much is immune to failure (it's just a dumb copper block!)
- a valve to regulate flow through the heater core
- a few sensors
Right now, the diagnosis is focused on the heating element
(I assume - though the title says "heater core", "PTC" and the part number refers to the element). That may or may not be the right one to look at.
To know for sure, you should probably check that the coolant is actually flowing (pump; valve). It flows at all times the car is running, in my experience. So, you should be able to pop the cap off of either reservoir, and see coolant flowing in both. Don't worry, it's a closed system - popping the cap while the car is running won't leak anything out at you, and you should be able to observe turbulence in the liquid inside. No flow? Check your pump.
Next, check if either reservoir gets warm when you've got the heater running - kind of unlikely at this point. Turn car on, set to 90, foot, no A/C, fan on 2. It ought to get warm within 30 seconds to 1 minute.
Next, check the "energy monitor" screen and see if you have any AC/Heating power consumption on the screen. Yes? Then something is heating up (and the heating element is OK) - as that screen monitors actual load. But through those other tests, that ought to have been eliminated...
Then, you can take the diagnosis to eBay, pick up the right part, and quite possibly fix it yourself if you know how to Google the service manual for the 2011 Leaf.