Replace the thermistor cartridge. It has extremely fine wires and they've broken.

It uses the same "paper scrape" technique that the TrueLevel does -- you do this step after the corners are level. My firmware uses the LCD panel (I have a ton of Viki1's...). It moves to 12 points (3X 4Y) and you use the jog-wheel to move the nozzle up or down until the "paper scrape" passes. When ...

And that's why my most recent firmware for the M2 uses a 12-point manual mesh level.

If #define DISABLE_E isn't doing it, then I'm going to have to go back to Ed's suggestion of you're pushing Marlin a little too far out of its comfort zone.

Tinned wires will creep over time in screw clamps, leading to failures.

If you set the output to 100%, it is "on", not PWM'ing. I don't know Marlin's behavior if you disable the temp sensor and command it to heat, though -- that's why I suggested the sensor 999.

They are. Make sure you use wire ferrules when adding wires to them. DO NOT USE TINNED WIRES.

It's OK here. I like it -- I have that issue, two M2s are a bit snuggly on 4ft shelving.

You want pins_RAMBO.h, or a MUCH newer version of the firmware than you're using.

Your "no thermistor" and "dangerous moves" will probably be fine.

You might be able to use Marlin's "DISABLE_E" configuration option, which will disable the stepper driver immediately when it's not being used. You can then swap the E0_ENABLE and HEATER_0_PIN numbers in pins_RAMBO.h. Make sure your TEMP_SENSOR_0 is 999, and TEMP_SENSOR_BED set to 998, and DUMMY_TH...

Use your other option. There are 3 sets of wires going into RAMBo's power supply -- the top set (nearest the blue fuse) are -/+ heater, then the next two are -/+ heaters/logic, and the last two are -/+ for the motors. Remove the middle two wires and use your own linear supply. That said, I'm on Ed's...