Great article, filled with a lot of useful information.
I will skip the formulas parts as usually :P.
Using a heat pump
Yes, something I am familiar with. There are many different heat pump systems. Like air-to-air, more commonly called air conditioners. Water-to-water, one of them is described by you. Water-to-soil, when you have a very long tube, grounded and in this way the heat transfer is made. The last one is what I think of, for my house, combined with an air-to-air one.
Water to soil is the cheapest system since it doesn't need to be dug very deep as long as you have a big enough surface. I have explored it a bit but for my needs I would need more than 500 square meters of area. I don't know if you can layer it multiple times on top of each other, this would reduce the surface area needed. And I don't recommend using the AC unit year round, no matter how good the inverter is.