As for Alexandria specifically in terms of regulations don't take advice from a public forum - look up the laws yourself and make sure you understand them.

By law, the most recent purchase price is the default value and it is impossible to overcome unless the property was destroyed after the recent purchase.

Well, I guess this says it as well as I could.This growth in ADU establishment goes hand in hand with progressive changes in Michigan's zoning laws.

So if, for whatever reason, laws change and you can't rent it out on Airbnb, etc, it still cash flows if you need to rent it out to a traditional long-term tenant. 

Hoping that my brother in law will be able to build and manage a contracting team. 

There are a couple of unique tax laws to real estate investors - 1031 exchange, bonus depreciation on certain improvements, Qualified Opportunity Zones, real estate professional status, etc.Make sure that the accountant you interview to understand the above concepts.best of luck!

My daughter and son-in-law have settled down in Abilene.

So I could hypothetically include a studio apartment above the garage and rent out the main house and be within the law? 

Florida is a unique state for MHP law and the insurance has gotten very expensive there too.I recommend checking out @Frank Rolfe's mobile home university content if you haven't already.

Duplex can always add additional units via the ADU laws. 8) Related to number 1, purchasing a property with an existing ADU is cheaper than buying a property and adding an ADU.