Have you tried contacting the main office or different individuals in the company? 

During this time you are not generating any return from the money invested in the ADU.

Gray area interpretation up to individual courts which have rendered seemingly contradictory decisions.  

In this situation, can all the partners claim mortgage payment as expense in their individual taxes (or just the partner who owns the mortgage)?

I've included an example below to help illustrate this.So different lenders have different rates (which do vary even for DSCR loans) but these are factors they all consider.See example below:DSCR < 1Principal + Interest = $1,700Taxes = $350, Insurance = $100, Association Dues = $50Total PITIA = $2200Rent = $2000DSCR = Rent/PITIA = 2000/2200 = 0.91Since the DSCR is 0.91, we know the expenses are greater than the income of the property.DSCR >1Principal + Interest = $1,500Taxes = $250, Insurance = $100, Association Dues = $25Total PITIA = $1875 Rent = $2300DSCR = Rent/PITIA = 2300/1875 = 1.23DSCR lenders generally let you vest either individually or as an LLC.

Requiring the homeowner to replace the pipe individually would seem like a huge octopus of inspections and enforcements and etc.

So, you, as an individual, may not get to the same rate that a MASSIVE national builder can...but you can certainly offer credits to the buyers to help buy down the rates. 

Generate money fast, within 30 or so days.

Instead of tying up your capital in a high-cost primary residence, consider taking the would-be down payment and investing it in out-of-state properties.This strategy allows you to diversify your portfolio across multiple assets, spreading risk while potentially generating cash flow and building equity in appreciating markets.

I like how even though you rent to the working class individual and they aren't $100K+ homes.