You can only detect Volts not how many Amps the circuit can supply.
A common way to think about electricity is to use a comparison with water, Volts is like water pressure so can be easily measured, but Amps is like how much water is flowing and until you get to the point where you can't draw any more water through the pipes you don't know if you have thin or fat pipes supplying you.
For car charging the EVSE tells the car how much current the car can draw, but that is just based on what type of EVSE it is. Some of the Level 2 EVSEs even have switches to set what the car should draw based on the circuit it is wired in to. For the Level 1 cable it is set to 12 Amps as I explained earlier and with instructions to have the EVSE as the only thing plugged in to the circuit.
The Level 3 chargers reduction in current at 80% is because that's what the battery can take so it slows down at that point, nothing to do with the Level 3 charger detecting how much current is available upstream from it.
Theoretically you and the Leaf owner could open up your L1 cables and reprogram the little computer in there to tell the car to only draw 6 Amps all the time, but that would suck any time you aren't both plugged in at the same time. The only practical thing is to get the two EVSEs plugged in to two different circuits, best of all would be to put 2 L2 EVSEs in the condo.