What I don't know is what "usually" happens with new home values in the USA.Are they like cars where they devalue the moment you leave the lot or is it realistic to expect they will hold their value and appreciate over time?

Are you ever worried that extra housing could be easily built in the surrounding spaces to devalue your property? 

Not much you can do about the Japanese yen being devalued or some such thing5) short answer is no, though there are some exceptions which are convoluted and not worth going into at low volume. 

Also, I read that power lines typically de-value a property by around 10%, does anyone know if that seems true?

They will rent higher and appraise much higher.Once you put a mobile home on land it devalues by 50%, the cost to install will NOT be the same as value to sell or what a residential appraiser says it's worth.Hard money is only going to be for 18 months and they might only give you 65% max of valuation NOT the cost to construct.

If you take into consideration the devalue of a $ that took place since 1950, what real numbers   are we talking about ?

Selling pipe dreams and telling people they can fire their boss and becoming super wealthy in a year is why we have feckless and untrained wholesalers all over the damn globe devaluing everything.

But because of that you can generally purchase it at a devalued cost.Many times the flood maps are inaccurate on that parcel of land. 

My plan has always been to have these properties as long-term holds, and I know selling this early (purchased in 2019 at 3.25%) and with such a low interest rate goes against conventional wisdomw, and I dont really want to, but as I learn more about where the US is in the long-term debt cycle, the more nervous I become about a catastrophic devaluation of the dollar, job loss, civil strife, etcCan anyone offer any advice for how to model or weigh the options on something like this?

Misogynistic behavior is not only blatant discrimination, but the includes the dismissal, devaluation and ridicule of opinions voiced by a female.