Ok, so there are a few things to consider here.
The first is pressure drop. With your rig, each pipe will have the drop of half of the total length worth of 2 inch pipe. I'm guessing 9 feet worth at 2 inch plus 9 feet at large area equals close to 10 feet at 2 inch.
The second is system balance. Uneven heat distribution means a lot of ground temperature water to extract the first time through. So with a series rig, you have a liquid thermal battery in the ground. As is, the system is good for intermittent operation at high efficiency. Once the loop is depleted, efficiency will depend on how much heat gets pushed each cycle. One can guesstimate a ballpark figure for the ground loop, but the truth will be hard to predict.
I'm sure you could fabricate caps for the big pipes a whole lot faster and cheaper than finding custom caps to make these enormous cold fingers. I'm wondering how much heat will transfer at slow and low flow rates versus turbulent flow. With the long time constant, the loop may not need stellar heat transfer.
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