I've heard some instances where the City allows the homeowner to get away with a sprinkler in only the new unit, but I don't think you can bank on that.Drawings/Permits - The City will require the new unit to be permitted, and may require a full set of drawings and plans.

For instance, consider two properties on the same block that are virtually identical but one has a carport.

So if I were to use the famous phrase from Godfather, "make them an offer they cannot refuse!"

For instance, if he provided an SOP that the PM had signed off on that specifically stated the only applicants they are to reject out of hand and that all other applicants would be considered?

I don't have good 'rule of thumbs' for you there on a per unit (for instance per foot) cost. 

There is however case law to go by from previous instances.

For instance, my mom-in-law fell last June,  hospitalized, and stayed with us till 2 weeks ago.

Nancy Zhao Depends- in many instances property values fell dramatically.

In the instance of a BRRRR investment, and the owner is able to cashout his principle up-front, would it seem fair that (assuming a 15 year note), I would get the same proportional share on the property once it's free & clear in 15 years or if the house sells?

I know that many people who "house hack" call themselves investors, but I realize now that because they live in the home they are renting out (a portion of) they are not really investors, they are just tenants who are in the process of buying the home (at least in that particular instance with that particular property).