Hello: There could be some interesting physics going on here 🙂 The only serious mode of failure I’ve heard about with Marathons has been overheated elements weakening the tank wall and creating a big leak. This was a problem early on and was “solved” by thermally fusing the elements. Air getting into the tank was the root cause. In this case you would know if air were getting into the system as you’d get spitting at the hot tap. Another thing going on here with pressure is the intermittent reduction down to zero or less, causing damage (maybe) to the tank lining.
There are two things to do. First is to verify what the pressure is really doing. There is a gauge made that’s somewhat rare, in that it not only measures pressure but also vacuum. Here are some; http://www.mcmaster.com/#catalog/117/561/=e8l03w .
I’d like to see one of those installed at a heater (could even simply be screwed to the drain valve) and then horses watered as per usual. You would then know just what happens pressure-wise. Next, if pressure falls too low would be to install spring check valves and also expansion tanks downstream of the check valves. The reason for this is two-fold. If you’ve got a check valve, now the system is closed and subject to thermal expansion, which can be harmful. Second is that the expansion tank would keep the heater under pressure as long as no water was used in the house until after horses were sparkling clean and no more horse water was being run. The check valve/expansion tank combination would do a lot to limit pressure excursions in the house and guest house.
The only other “fix” that comes to mind is to limit water flow at the horse barn to something lower than the well pump can deliver. This could be done by measuring pressure at the house while running water in the barn. Installing a valve to throttle flow at the barn would limit pressure drop in the house. I like the check valve solution better, but you have choices 😉
ps. You might want to run dehumidifiers in and below the guest house. This can help with flattening out the floor.