It's been a little while since I started this thread. I've done a lot of research and learning since then.
Here's how it's going to work now:
I'm getting 1-1/4" O2 barrier PEX and spray foaming that into the ground. The fittings aren't that hard to find actually.
The 1-1/4" lines are running into the house into a custom manifold I had built that has a 1-1/4" trunk, two 1" outlets, and five 3/4" outlets. I'll be drawing my water/air heat exchanger heat from one of the 1" outlets through 1" PEX because the manifolds are literally 4' away from the heat exchanger. I'll be running my domestic hot water through a sidearm heat exchanger, with one of the 3/4" outlets supplying the heating water for that. As of right now, the remainder of the outlets will be unused, but valved just in case I want to add a loop without shutting anything down.
I will be putting a domestic coil into the OWB still. I'll run that directly to the garage via 1" lines. It will have it's own pump on it, with a heat exchanger and expansion tank in the garage. It will be filled with antifreeze. I'll be able to shut that loop down when I'm not in the garage without worrying about freezing, and I can install it at any time without shutting down the rest of the OWB setup because the domestic coil feature will already be installed in the OWB.
For PEX connections, I will probably be using crimp fittings. I did some plumbing in my house with PEX recently and I loved the crimp fittings. They look better than clamps and are far cheaper than sharkbites.
My father in law has been an HVAC guy for 20+ years now. He looked at my old ductwork and decided it was crap (only setup for heating and way oversized for my house). I tore it all out. He designed new ductwork that fits into my old house and is meant for heating AND cooling. He sized all runs to keep up air velocity so all of the rooms actually get heated or cooled. And, he's installing it all as well. For everything including new vents and registers and returns, it ended up being less than $1000 since he got it all at cost through his job. He got me good working take-outs of a heat pump and variable speed air handler for free. Now I'll have a heat pump as emergency backup heat, and also air conditioning in the summer, and a very good air handler that we've tied my 22x22" water/air heat exchanger into.