For A Retrofit, I would say no, but since it's a new installation I would just toss it in anyway. While it's true the payback will be longer the equipment should last 2 to 3 times longer since your not utilizing it 1/2 to 2/3 of the year.  This also gives you the flexability in the future if you decide to make it your year round residence when you retire. That's My opinion anyway.

If a closed loop system is properly designed and doesn't have any leaks, it should be able to go for years with no maintenance other than an occasional check of the pressure gauge. Should a pump have to be changed they run about $200-250 (retail) for the units typically used in residential systems. If it is plumbed properly with isolation valves, changing it should be a 15 minute job and they system might not even need purging. I'm also not sure why bronze pumps aren't standard - they only add a couple of hundred dollars to the cost up front and should last virtually forever. Again, if there are no leaks in the system it shouldn't need to be purged periodically. Some systems are capable of purging small amounts of air on their own anyway after initial setup.

If the antifreeze is glycol based it might benefit from a more frequent check-up to monitor the water chemistry. I'm not that familiar with glycol but I've heard it can be a pain to work with and does need some maintenance. Most hydronics guys avoid it like the plague unless it is absolutely necessary. I've not heard of any problems with methanol-based fluids.

All in all, there is no reason a water loop should present a reliability or maintenance concern. It is a very simple part of the system. Problems that people have experienced here are likely due to poor design and/or installation. I think a lot of Geo installers come from more of a forced air background and are at a disadvantage when it comes to pumps, piping, etc. Some of the "industry standard" design practices for water loops make no sense imo. Look at the hydronics industry - there are hundreds of thousands of radiator, baseboard, and radiant floor systems that circulate water and need no periodic purging or maintenance. It's not that hard!

Yes, a catastrophic failure of the ground loop or pump would casue a sudden shut-down, but you can say that about just about any part of a geo (or any HVAC) system. Compressors, heat exchangers, control boards, blowers, etc can all fail catastrophically and without warning. These are rare, and with good design and installation I think water-based or DX systems are generally as reliable as most HVAC systems.

DX and water based systems each have advantages and disadvantages, but I wouldn't base your decision on water loop maintenance issues. For most people they don't have to be touched for years and years.

Bear in mind that I have no particular expertise in GSHP. I've just been reading furiously over the last year. The concept is clear, SCW is a proven design, and it seems that only water quality would stand in the way. In trying to envision how a DX tube in the well would work, if a longer bore were required to provide the necessary heat transfer area from the rock, I first thought that since a regular SCW design would have water circulating back to the top of the well even when there was no demand for water in the house, then why not have a DX-in-the-well design where there also is water circulation when the heat pump runs, from far below the bottom of the DX tube to the top of the water column. I don't know how the controls work in a normal SCW installation, how it switches the pump from low-head to higher head when domestic water use requires it. Maybe the circulation diversion could occur within the well, or maybe it has to occur inside the house, with a return line to the well. Perhaps your thought of a second pump loop, entirely within the well bore, a fairly low head pump, would be easier to devise.

Am I getting the impression that there is no "standard" system design, that all sorts of flow and control schemes are cobbed together for a particular installation?

However you wind up doing the heating system, I do urge you to put a lot of effort into designing a very well insulated house. Even if it turns out you can't easily do GSHP, at least the house will be noticably more comfortable and more isothermal and have fewer cold spots, if you don't just build "ordinary."