Glort, I'm confused about your goal re: smart meter. Why not just get a meter that allows you to legally backfeed. Sure, it's not as good as your bootleg setup but you would offset your own use completely on sunny days so it's not so bad. Or is that a problem with licensing, inspections, etc?
For the direct DC water heater, some custom electronics may be required that may be beyond your comfort zone. Let me sleep on it and see if I can think of something simple. Using the existing heating elements means needing close to but not over 230VDC.
Wish you had a schematic or specs for the PWM unit; no telling how that would fair with HV DC input otherwise. I am not aware of high voltage and high current DC-DC converters but I suppose they could be out there. I know some big UPS units had battery voltages up to 144V nominal. Take a look and see what's out there.