Your definition is correct for "legal non-conforming".The "legal" part of "non-conforming" is dropped when it doesn't apply so most commonly people call these units "non-conforming", "finished", "in-law suite/unit", or "bonus unit".Non-conforming: Any property that doesn't meet current zoning requirements.Legal non-conforming: A non-conforming property that was lawfully established under previous zoning regulations and is allowed to continue its use.Based on his posts @Mario Morales is referring to a non-conforming unit.

Use some for buy and hold (I personally would prefer apartments rather than buying a few single family homes) and some for lending/trust deed investing.In either category, you must know what you are doing, know how to vet the investment, how to vet the sponsor (if investing in a larger fund of some sort), the laws and regulations on the lending side, how to investigate which areas to invest in buy and hold and notes/trust deeds for that matter.So as you have already said several times, lots to think about . . . 

There's plenty of regulation surrounding the STR industry, but it's perfectly legal and welcome here.

We will continue to have an influx of transplants from high tax, high regulation states like CA and NY. 

I can't compel competence that does not exist.

There's a to unpack here - preparing the property, pictures & listing, finding a management company, taxes, regulations for the area etc.

Also, if you have anything in your existing escrow account, you will get that back within a few weeks of closing.

When analyzing a new build/“build to rent” property as a rental, what percentages/amounts are others using for:-Maintenance/Repairs-Vacancy-CapexI’d assume it’s less than than existing homes, but by half or is it safer to use the standard percentages?

The existing homes are too small and would require a large expansion, at which point it would just be better to build new. 

There will always be bad actors, you will never regulate malintent away.