I'm not entirely clear what you are asking. If the water table is 3' from the surface, I agree with your neighbor...a basement is a bad idea.
Concrete is porous so in and off itself will do absolutely nothing to stop the water. You can waterproof it by applying a rubber based membrane to the outside, but with a high water table, it will always try to find a way in. You'd have to definitely install French drains and a sump pump as well.
I'm wondering if the problem isn't mineral buildup, but rather sediment in the lines. Not sure if this would cause a failure in your softener, or if it could be caused by the softener failing.
The sediment in the pipes would go to various fixtures and get stuck in the filters and aerators. You'd have to remove each one and clean it out. On sinks, this is usually part of the aerator and may unscrew without major tools. On shower heads, this may be where it screws into the elbow pipe. And on toilets, newer ones have a cap at the top of the supply pipe that you push down and give it a quarter turn (turn the water off first, with the cap off and a cup over the top to keep water from going everywhere, run some water to clean it out, and then reassemble).
For the hot water heater, you would need to simply flush it out (which should be done periodically due to natural buildup). Attach a hose to the drain on the bottom, either run it outside or into one of your drains, turn the heater off so you aren't trying to heat water that's being discarded, and open the drain for a bit. If you're observing the outflow, keep going until it runs clear. Otherwise, I'd guess 5 minutes of draining every year should be good, but that's a guess.
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Up to you, personally, I just fitted it and carried on as normal.
But you can do the flush as you wish - I still have deposits but my water is very hard, however much reduced. Once a month cleaning the kettle is now once every 4 months or so...