If you feel you can bill separately for water if metered you should have no issue simply adding a flat water fee and I’d imagine the $17,800  could go a long way in a 2 unit building for upgrades that would have a greater impact on revenue. 

-How is this any different than simply leasing out individual rooms, or a guest house in a single family home? 

Lenders can use legal ADUs to help qualify for the mortgage, so people that didn't even want to house hack before, may be in the market to do so simply to afford the type of house they want. 

Maybe the market is hot enough that you could still turn a profit selling as is, but typically, sellers will ask you to take care of them before closing AND likely pay less for the property simply because there are known defects.

You simply cannot rely on Tenants to accomplish this.

If you where going to be able to recreate the orginal layout, you would of been able to do so by the clues left behind by remaining framing, be it the "honey" holes where the toliets once where, the sizes of the holes in the joist would indicate what pipes ran where be it hot and cold supply, toliets, showers and other industry standard tell tale clues. 

In over-saturated areas like Scottsdale, hosts are often asked by guests to discount their nightly rates (especially for extended stays) and many hosts accept the discounted rates because they know that if they deny the request, the guest will simply book with a different host that will extend a discount.  

If you don't wish to add, you are simply being a troll.

There is no answer, there are simply alternatives. 

They tear it out when they leave, and leave you a big hole.