Just get it tested and a filter and softener helps the taste and hardness we have here.

Buyers are adopting a "wait and see" mindset, so you may find the market softening significantly, with many buyers postponing their home search by the time you finish your flip and list it. 

The housing market is very hot now and there is a good chance that the market will soften soon.

Belleville is not too far, also Garden City and Ypsilanti.Could you househack to make the place more affordable and perhaps soften the blow of a high-ish sales price?

I think I could sell it for 200k, but there's no guarantee based on how our market is softening.   

I assumed an extremely high CapEx over 18 years (2 washers, 2 dryers, 2 fridges, 2 water heaters, 2 water softeners, 2 dishwashers; 1 furnace and 1 AC unit, and 1 roof).

……the water softener was constantly cycling causing insane water and salt use.

It's better to not buy anything if it doesn't meet your criteria rather than regret the purchase.When the market softens and turns (we don't know if it's this year or next year), you'll be glad that you waited!

Manufacturer recommends nothing but does anyone think I can add a little extra protection, insulation, and soften imperfections in the subfloor?

Over the past two years the area has really appreciated rapidly and now I estimate I can sell my home for $1.6-$1.7 (our neighbor sold for $1.8, but the market has softened over the past 3 months).