Good evening Mike, awesome job, and I bet lots of fun too.
When I lived in Haast, the dude that owned the motorcamp had some water heaters made. These were a double skinned stainless drum, water circulated in the perimeter gap producing prodigous amounts of hot water burning with driftwood from the beach as fuel.
Another one that took my fancy I saw in Ozz, this design modeled on the old NZ railways thermette, only bigger. The "cone" held around 20 gallons of water, this boiled inside 20 minutes with just a bucket of wood chunks, It was around 1 meter tall and maybe 400mm at the base, chimney exited the top.
Coal and wetbacks are as common as mud here on the coast, yours is very good and will work well. Just vent the hot water cylinder really well, you will have an excess in winter.
A funny story proving we can all learn.
Neighbour showed me his wetback installation.
That wont work I said, the 44 gallon drum as your hot water tank will rust away and the plastic pipe you have used to plumb in the hot water will melt with the heat.
No it wont he said, and dismissed me.
Weeks later I went over again. The plastic pipe was now rock hard with repeated heat cycling, easily as good as a metal pipe, and the drum had no rust, hot water has no free oxygen to corrode anything.
Twenty years later, its still working fine.
Mike, you have over designed this.......