The biggest factor will be how long you plan to own the property or when you think you may refinance.

An LLC is potentially one minor factor that might contribute to the evidence that you are running your rental activity as a trade or business, but it's definitely not required that you have an LLC to qualify.

Their contribution depends on factors like local demand for ADUs, their design and size, and how they’re used (e.g., rental income potential).

I own a small multifamily commercial in Albuquerque and can confirm the diversity of the economic and cultural factors that you mentioned. 

Do the calculations factor in default rates?

We measure against 15 different factors.

Just factor in a little time and effort to get everything squared away, and you should be fine!

What you paid for it and invested in its upkeep are not factors for pricing/selling a property. 

DSCR loans don't require all of those things - your FICO and the income from the property are the main determining factors.

Have multiple exit strategies, do not be over leveraged, and make sure you are factoring market changes.