What pin is the TPS, power and ground, on the ECU?

WAY faster to borrow a scanner that reads live data. If the TPS signal voltage sweeps from 0.5 volts to 4.5 volts, the entire circuit is okay and there's no need to go off on a wild goose chase. The better scanners will also allow you to run the idle speed motor up and down to test its electrical AND mechanical operation.

Will the engine stall if you don't keep your foot on the gas pedal?

Measure the voltage on the center pin of the TPS. You should see it go from 0.5 volts to 4.5 volts from idle to wide-open-throttle. If you find it stays near 5.0 volts, the ground wire or terminal is open.

Actually, it would be faster to just measure all three wires than for me to type all the possibilities. The ground wire should have close to 0.2 volts and the feed wire should have 5.0 volts.

There's two ways to set code 123, voltage too high. One is with an open ground circuit. The other is with an open signal circuit. When that happens, to prevent the signal voltage from "floating" to some random value due to other circuitry in the computer, and confusing it, a "pull-up" resistor is used to force the defective circuit to go to 5.0 volts which will set a code.

If you don't get that nice smooth sweep from 0.5 to 4.5 volts, you can test the sensor with an ohm meter, once you know which pins are the ground and the 5.0 volt feed. Measure between them to check for continuity. The exact value is not important but I'd expect to see somewhere around 5,000 ohms. Next, measure between the center signal terminal and either other pin. You should see a smooth change in resistance as you run the throttle open by hand. You're looking for a dropout, where it suddenly goes open circuit, (over range on the meter). If you see that, replace the sensor.

An open wiper inside the TPS can cause that open signal circuit too. Typically that is an intermittent problem, but once the code sets, the computer knows it can't rely on the signal. That could explain the failure to maintain idle speed. The computer might have to relearn "minimum throttle" before it will know when it must be in control of idle speed. Chrysler and Mitsubishi have a lot in common so the relearn procedure might be the same. Drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals. The same memory relearn is usually necessary after the battery has been disconnected or run dead. That relearn probably won't take place unless the code is erased first.