Here's your performance data:
https://packless.com/drawings/coil/performance/Classic
For as cheap as you paid for the one coil, you could buy another and gain maybe another 10 - 20 percent COP, and still spend less than the retail price on a new HX! With the operating parameters you posted, this coax coil should do as well as a prefab GSHP. Remember, though, that the HX must work against a decent DeltaT to transfer lots of heat. If there is not much difference of potential, the exchanger won't be very effective. More water flow is not always better. The literature shows the relationship precisely for your HX.
As far as oil, I wouldn't worry too much about it. If you didn't just poke a big hole in the circuit and spew foamy froth everywhere, most of the oil will still be in the compressor. Window units don't contain much extra oil in them, as the refrigeration circuit is small. The only oil that will leave the old unit is whatever tiny amount sticks to the walls of the old HX you remove.