The insurance company bases this tiered cost on the % of Building Replacement Cost insured.In your example, if your duplex is 2,000 sq/ft and the 4 plex is 4,000 sq/ft, then your assumption is correct. 

@Peter XiaoI have no idea what is going to happen with ratesI will keep buying on the assumption that I will never be able to refinance, and I will make sure deals pencil out based on the rate I lock

Basis of my assumption: My house was $320,000 in 2020.

I would operate under the assumption that the high desert is a short-term friendly location and the regulations that have been put into place were put into place to protect the investment of short-term rental property owners, the tourists, and full-time residents alike.

Independent Bank, Southstar, etc..Also, look at finding a Nonqualified Loan Assumption deal.

I found a home in Napa at Silverado that I could buy with a simple assumption ( SF was in the tank) it was only 10k down and 10k to transfer the golf membership.I owned at the time my current home in Palo Alto.

When you consider all of these things and start with the assumption that you need a place to live, House Hacking often comes out as a clear winner even if your "cash flow" is actually negative but you are paying less towards a mortgage than what you would be paying towards rent. 

Are these assumptions correct?

Let's also operate under the assumption that the vast majority of people do not live in the same property for 30 consecutive years.

I think if the partner cash's out now  I am making the assumption that he is just not quit claiming for no consideration he will owe tax just like the 1031 never happened..