And the lines to the hot water heater run below the engine and then back up to the water heater. I've never had an air problem. The engines water pump has plenty of power to push coolant and air through. The air comes to the top under the cap and I fill until it's set.

I don't know about the heater but I look forward to hearing how it works for you. There are two big schools on heating houses or boats. One is central heat, the "furnace", ducting or piping to radiation. The other is a central heat source both convection and radiation, the "stove".

You could write two books on why one is better than the other. I prefer the simple myself but my boat has a small enough central area where a radiant stove source is very cozy. And the stove heats the head as well. On a bigger boat, it might not work as well.