I have seen this same approach being taken by people selling bars, where they own the property as well.  

After the BWI Meetup people are usually at champions bar in the hotel until they close at midnight

Since we intend to be the occupants for that space we don't need a full kitchen, essentially we just need a sink so what distinguishes an ADU from say adding a Wet Bar to a basement?

Remember the worst tenants are usually the best con-artists.Stick to your criteria, if you have plenty of applicants raise the bar in the areas where you can in choosing the "best" fit and never lower standards to try to make a otherwise unacceptable applicant fit. 

It seems to me that you have a great potensial to achieve your goals as long as you always push the bar for your ability and desire to save.

Or barring that possibility, a way to use financing while insulating our credit from each other so that if the worst happens, my friend disappears, and I can't cover her share, I'm not totally screwed? 

How much will it cost when all your tools are stolen because you didn't have bars on the windows?

When you're in low-income areas you have to set the bar a little lower.

:https://www.biggerpockets.com/forums/52/topics/680...Without providing a laundry list, I think the theme is to tackle the wear points and troubleshoot the individual unit.For example, start at the door (door mat to catch debris, good functioning new lock, perfect close and open, door stop) and work through each room with that level of detail ( example: Bathroom: new shower curtain, bath mat, caulk, new toilet flapper or overhaul kit inside, strong towel bars, etc.)I would focus first on the low cost items that can prevent larger problems (your doorstop type theme) and then also evaluate any larger item close to the end of its useful life, especially if showing signs of problems (be it hot water heater, worn flooring, or an appliance). 

The kitchen will also will have wet bar, mudroom valet/cubbie.