Take a look at AirDNA, the enemy method, STR Insights to get a better idea. 

Now I think your specific question is how do these cap ex items change over the evolution of construction and are there certain dates that we might see a change in method.

Photos of the land would be helpful especially if you want to price the digging method as well as the burst method.

Several methods come to mind for the ADU.   1) a regular or periodically adjusted flat fee (based on an estimated usage); 2) factored into the rent (essentially the same as a flat fee); 3) percentage proportionally based on sqft. 

Yes - should be enough if that is your budget, especially if you are doing the BRRRR method which it sounds like you are doing

My only question would be are you open to implementing the BRRR method with this duplex or are you looking for something move-in ready?

Quote from @Tom Sharon: There are many ways to address this, and I'm not familiar with your state laws.The simplest method: post a written notice on the door.

I do question how applicable that technology and method is for market rate US product. 

Probably has a better chance for higher cash flow in certain markets with that method as well

It's always wise to assess one's financial readiness before committing to such a significant investment.So whether you go sub2, seller financing or some other method, its all debt - just a matter of who is providing it.