Ok, I will answer a question at a time for you. These answers are not going to be thorough or exhaustive.
First, the water heater idea is not an especially good idea. Yes, it will keep your heat pump from freezing its water loop, but it is blatantly inefficient unless it is running off solar power. You are basically warming water just to cool it back off. Find another backup heat source below the balance point.
Second, there is an endless supply of flow control valves on the internet. The main idea is you usually get what you pay for. The cheap plastic fleabay items are ok to experiment or prototype with, but for a final version it is much wiser to install a piece you know will be durable. I like the ASCO "red hat" valves and their copiess. Check out surplus city liquidators for some awesome prices and customer service. They have a massive selection if items, so do a little research to figure out exactly what you're after. It is easy to get lost in their site, and the descriptions are fairly vague on purpose. When in doubt, call the store. Their phone people are expensive at other places.
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