My understanding is that an ADU would be converting non livable space or adding livable space not just changing an existing SFR

I do not know anything about your or its location but you might look into an apartment complex as a possible investment if any existing complex is available for purchase. 

Is the market for this saturated in this area or as long as you have a good place you should be fine?

The best and highest use might be redevelopment instead of re-stabilizing the existing asset use.

My moving existing Roth funds in this fashion, you are negating the ability to add new Roth funds to your overall portfolio.

Let's say the existing tenant gives 30-days notice on 7/1 and will move out 7/31.Immediate next steps...1.

Not sure if AirBNB can exists outside of the strip.Terry

That's partially why MHP-focused mediums like BP, conferences, podcasts, etc. exist.

Would the drainage requirements be mitigated by making the parking lot from pervious pavement/asphalt and the installation of a green roof(which would contribute to stormwater absorption?)

If the existing lease does not have the language in it that allows for that kind of extra expense (which is probably unusual) that makes a tenant partially or fully responsible with explanation to the new tenants responsiblility for that type of expense for air conditioning that is unusual unless common in that area and might be discovered during the research period on the subject property and uncovered then.