PVC, PEX, or Copper?

Ace Hardware carries an extensive line of connectors and fittings called "Sharkbite" (I think) that just push onto the end of a piece of PEX and lock. I just used one on a stool in one of our bathrooms last weekend. There's also a tool to release them if you ever need to.

in the barn, I used the black poly used for underground sprinklers, along with the corresponding barb fittings and 13/16" hose clamps. Used conduit clamps to secure it to the walls. Up about 2.5 years with no issues.

Your water heater idea is used all the time in motels and hotels to keep instantly available hot water to all the rooms. Your returns would have to be right at the outlet valves to avoid them freezing. Costs to heat the water but heat tapes are so expensive and use so much heat and I don't trust them. At 0 degrees, any cold air getting to the pipe in a particular place could freeze them anyway even with heat tape.(I lived in a mobile home once, BTDT) Heating the water oxigenates the water and to me doesn't taste the same. Horses are kind of finicky about what they drink. Put some hot water in a bucket, let and cool and see if they drink it. I have two horse tank water lines (water hose actually) that loop overhead and come down to the tanks with a 2 line splitter valve at the first tank and one end at the faucet. Unhook the faucet and open the valves and it all gravity drains. If I had a lot of horses and tanks, I'd go with a more permanent setup. Before I got my hoses looped right to drain, If one of my hose sections froze up, I'd just take in in the house to thaw. Main problem is to tie the ends down and the tank heaters down cause horses like to grab them and throw them on the ground.

I don't think that would keep your drops from freezing.....