12/2 may require a 20A breaker
Note ThreePhaseEel's comment, where he notes certain motors require a 25A or 30A breaker (NEC 430.52) yet are permitted to use 12AWG wire (430.22 notably 430.22E), if so, this may have been legal after all. I would, at least, upgrade to 10AWG until it leaves the house. I care if it burns down your house. I don't care if it burns down your yard.
If the amp draw is well under 20A, then go ahead and fit a 20A breaker and now you're code legal in any case. And due to a happy coincidence of the numbers, your wiring run is almost exactly 1 ohm of resistance. That means the voltage drop equals the amps. It's a lot easier to measure voltage drop than amps: measure the voltage at the panel, measure the voltage at the pump, subtract. The pump must be running while you're measuring.
Yes, you can run 20A on 12AWG for unlimited length. Your load might not like the voltage drop, though. That's why I suspect the actual amperage is well under 20A.
Don't use a lightbulb for a heater. Use a heater.
Light bulbs burn out and you won't know when they do. Just go to McMaster-Carr, Grainger, or other industrial supply and buy a heater module. For instance they have a strip heater, 240V, 3 inch wide, 120 watt for $19. Any common strip heater will do fine.
Or even better, they also sell pipe wrap heating, which you can then wrap with insulation. Could take well under 100W!
There's no way to string 120V out there
The pump has 240V hot-hot-ground 3-wire electrical service out there. Neutral is not ground so there is no way to obtain a neutral to get 120V service. No big deal since you can get 240V heaters.
If you must do this thing with incandescent light bulbs, get 3 bulb bases and wire them in series to 240V. Then put 3 identical bulbs in there. Each bulb will see 80 volts (1/3 of 240v). Since that's 2/3 of its normal voltage, it'll run 4/9 (2/3x2/3) of its normal power. The bulbs will last a great deal longer, but when one fails, they all go dark.
My recommendation is to build a "float pan" under the run of Cu pipe in question. As Ed Beal suggested, don't depend on the sump pump as a diagnostic tool. The pan only needs to be an inch or so deep, as you expect it to be bone-dry all the time. Put a float valve on the pan, and tie into well pump power as planned.
BTW, I didn't come up with this on my own: central air conditioners' air exchange handlers have this sort of arrangement, to avoid disaster if the drain line clogs.
Best Answer
Read article 430.102 of the NEC for a description of why some well installations do not require a disconnect within sight of the motor. One of the examples used in describing the exception condition (a) is "submersible motors", which covers a lot of well pump motors. If you have a submersible, you don't need a disconnect at the top of the well, it can be elsewhere. If you have a jet pump or turbine pump where the motor is at the top, you do.
There is no general rule on minimum height, but there are several unrelated rules (mobile home and trailer pedestals, EV chargers, etc.) establishing 2ft from the ground as a minimum, so it's best to use that even though it doesn't apply. But if you are in a flood plain, you should pay attention to the "100 year event" level.