All of that makes sense to me.
Our installation uses straight pipes, with the return about 1.5 to 2m from the flow. I can see that straight pipes need a longer trench but that wasn't a major issue for us. We have the space for it and the cost of just getting a digger here was high enough that the length of the trench it dug whilst it was here was a relatively minor consideration.
One thing we did was to install 2 pipes instead of one so we have double the required length. The main reason for doing this was so we could easily add a second heat pump if needed, since the one we have is theoretically borderline for the size of the house (but has coped fine). Double pipes has the side effect that flow rate is reduced which should be more efficient for both thermal transfer and pumping. It also means if we ever have a leak we can re-plumb to use just one of the loops and avoid digging up the field again. For the comparatively small extra cost of a second pipe I think this was a good investment. Anyway, we made sure the 2 pipes were separated and never crossed, thus avoiding some of the issues you raise re coiled pipes.
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