Can You Run A 240v Heater On 120v

You are indeed spot on that your heater will draw 9 375a when connected across a 120v supply.
Can you run a 240v heater on 120v. A 120 volt will normally have a single pole. Because of ohm s law and the fact that the resistance of the circuit stays the same half the voltage means the watts of heating energy generated is reduced to one quarter of what it was at 240 volts. Prograde certified electrical contractor replied 2 years ago no a 50 amp circuit is not suitable for that use in that manner. For example in an outlet that uses a toaster or a blow dryer you would have to run one or the other with a 120v but with a 240v both can be used at the same time and time equals money.
However your 240v wiring is a miss. There are no adaptors. Yes you can wire a 240 volt rated water heater with 120 volts exchanging the second hot red wire for a white neutral wire to the neutral bus. Electrical circuit breakers should have amperage marked on the togle handle or on the top part of the module.
Keep in mind that the same power rating water heater can be operate on both 120v or 240v ac. As we know that the same water heater rated for 240v can be operated on 120v. To get 240v from a 120v supply you use a 120 240v step up transformer. If so the heater element may damage or burn out which may leads to fire.
Contrast with a properly wired 240v circuit which can carry 3840 watts or almost 3x as much that is why baseboard heaters are mostly 240v. So it wouldn t matter if you got a big baseboard heater that is 120v you couldn t hook it up anyway. Clothes dryers water heaters and and other high power 240 volt appliances cannot be run. And your 120v wiring is correct as well you are indeed correct that the two water heater hots should be connected to hot and neutral of the 120v supply and the supply ground should ground the water heater.
Can the breaker be changed to a 30 amp 240v and the outlet run the 2 1500 watt 30 amp 120v heaters. Keep in mind that you can t operate a 120v water heater on 240v ac.