Yes, I do have a multimeter
I did the test that you asked. Let me sum it up:
Basic test for teensy:
- voltage on clock and data pins with no keyboard connected: both 5V
- clock and data to ground: the teensy (or usb cable for that matter) does not get hot
Basic test for the keyboard:
- Current of the keyboard (only with power): 0,024A = 24mA
- Data and clock voltage: both 0V, power dropped to 0,8V
- Current of the keyboard (clock and data to grnd): 0,097 - 0,1A = 100mA
- Data and clock voltage: both 0,02V, power also 0,8V
In all these cases the teensy and cable get hot.
I would say that there is something wrong with the keyboard then?