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Full Disclosure - I'm an owner of JWB Real Estate Capital (Jacksonville Wealth Builders.)

If I booked a place and I agreed to the cancelation policy, my plans changed and I do not get a refund then why even bother the owner.

How can I find a list of homes that are not currently for sale but that may have an ADU, so that I can start to reach out to these owners about any potential interest in selling?

Does anyone know about the new owner?

2 points - First, in most states (and SC appears to be one), you can only act as Owner/Builder on your own residence, not investment properties. 

Worst case scenarios are like you pointed out, or there is a disagreement of the contract and you friend backs out.When you are renting it out, typically the owner (your friend) is responsible for taxes and insurance. 

Will they allow a loan assumption as the new deed owner?

Once you have a list of properties that you are interested in, submit the owners names and their mailing address to a skip tracing company to get the owners' contact information.

I would not be too concerned about the STRs because from what I am seeing with the STR owners transitioning to LTR, most of them want to rent it furnished as a LTR and charge more than market rent.