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Yeah, somehow i managed to miss this.
When in doubt, more C4 !!!
Use relays as you have in the drawing for ARCOK, do not wire anything to Mesa, do a test cut and check if the relay closes while cutting. Do the same thing for TorchON but reversed, meaning contacts go to plasma and coil on the Mesa side but do not wire to Mesa just use whatever can give 15-24V and trigger the relay with it see if the torch fires. After this, you can wire stuff to Mesa, one at a time, test, wire, test....
As for the Arc Voltage, find the big rectifiers, usually with big aluminium heatsinks and find the positive and negative, wire two short wires to each, wire 2 of 1MOhm/2W resistors to each of those wires, on the other side of the resistors wire two thinner wires prefferably shielded and send those to Mesa THCAD, mind the polarity, although nothing will happen if you do mix the polarity but you will have no voltage reading on screen.
Make positively and ultimately sure you have the power cord and plug under your foot while adding this, i do that always to prevent someone somehow getting the idea that it belongs plugged to the wall...while i have both hands inside!