My personal opinion is that unless there is no other way to sell the house, subject to should only be used when BOTH parties are sophisticated investors.  

Or should I sell and jump start my real estate acquisitions?

Signing it knowingly and selling their short sale property to a family member is Mortgage Fraud.

Just remember if you sell it, make it your primary again by living in it for 2 years, or buy a new property using a 1031 to avoid capital gains.

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?

I'm a part of partnership that attempted to do a flip in Fl, but due to market conditions, bad contractors and multiple change orders, etc....looks like we'll be selling at a loss. 

Jack Bosch and a lot of the land flipper guys who teach it like to have their students buying in the high deserts and remote areas of the Western US  or in FLA like le High Acres etc. this is were cheap land is and land that gets traded constantly at tax sales.. the play for most of them is really simple.buy at tax sale or with direct mail.. for a few grand  re-market them for more and sell on terms and collect monthly payments.. 

They think like selling a used car they can say "as-is" and that relieves them of any liability.

A seller called us back and wanted to sell this property, made more sense to flip it.

Some lenders hold there paper, some sell it, some do both.