With the economy of scale in multifamily, each additional unit nets you a little more than the last, as a rule of thumb.

Connecting with local GC groups in your area will help you connect with subcontractors, find out tips and tricks of the trade in your local area (since all areas and their real estate differ slightly).Also, have you considered the motivation behind building a brand-new duplex versus finding one on the market?

This will all depend on how good of negotiator you are. 

If not, they may consider building the ADU and down-sizing into it and renting out the main house.

I actually got caught up in the creative finance bubble of trying to lock up properties way over their value just because it had creative terms not realizing that if you were to buy a property at 120% of its value you will never be able to refi or sell without paying out of pocket.

It's also easier to handle dips in census similarly to dealing with rental vacancy if you have one rental property compared to a larger apartment building.My wife loves the operations but we have steadily been working her out of the day-to-day stuff. 

However, if I purchased at today’s rates (or cash out refinanced it at high LTV), it would still be negative cash flow.  

And the split is whatever you decide on. 50/50 is what a lot of people usually do.

Of course!

I think your demographic might be more hotel people rather than families.I would also still go with smart locks on every door.