Does connecting neutral to chassis (bootleg ground) rise chassis potential to main's voltage with respect to ground?
You do not know Ohm's and Kirchoff's laws. Calculations with them shows the mains voltage against the GND (=ground) is distributed to 3 parts which are proportional the resistances in the series circuit. The heater has resistance about 22 Ohms. The hot and neutral wires have much lower resistances, say 0.1 Ohm each so the voltage between GND and the bootleg-grounded radiator case is only about 1 volt. That voltage counts if you happen to touch at the same time the case and some other grounded object. It's not dangerous, maybe even not noticeable.
Between the radiator side ends of the hot and neutral wires there's left 218 volts. That's the actual voltage which generates heat in the resistance of the radiator. Of course the 1 volt drop in both wires also causes some heating in the wires, but if your system obeys the code the wires stay cool enough.
It can happen that someone cuts the neutral wire by accident or there's a loose connection which makes the resistance of the neutral wire infinite. Then you can enjoy full mains AC electrocution by touching at the same time the case of the radiator and some other grounded object, say a water pipe or concrete floor. This is the reason why the protective ground wire must be brought separately from the main electricity service panel.