How serious should I take the cost approach to value of building the place new in today's currency and how does that potentially relate to a projected ARV? 

For stocks to ever reach 2009 levels, you'd need two things: 1) The most massive selloff ever seen by both big money and individuals2) Massive deflationMassive deflation could happen for a short period as long as consumers aren't spending...but with the strength of the dollar still as the reserve currency, it likely won't happen. 

“…if the market crashed, the value of the dollar declines dramatically (as well as other world currencies), what will the government do to secure the dollar?

Find the right deal in those markets, and pair with a hard money lender for the rehab.

We are not talking which milk to buy or pair of jeans. 

More likely it’s for building a pair of townhouses.

I went with the Amazon suggested option for the diverter to pair with the shower hardware, didn't double check, and it was incorrect.

My second question is would it be easier to rent my bedrooms as a pair, or each one separately.

Vacation Rentals and COVID-19 do not pair well. 

I think the article is right on when it says, "the city’s biggest advantage is that it pairs affordability with growth".