All good questions Sky,
I'm sure you've seen that there's plenty of debate on all sides about these issues.
Most of the people I know using auto fuel use a mixture of it (ethanol free) and 100LL and seem to have good results.

My concerns about using auto fuel have been documented ad nauseum elsewhere- cleanliness, storage, effects on "modern" aircraft propulsion systems- but I have no qualms about using auto fuel in my Cub as its engine originally ran on that type of stuff.

Ethanol, however, is kind of unproven territory to me based on everything I've read, and the fact that the STC specifically prohibits it. I'm not sure the prohibition is based solely on the effects of ethanol on parts of the fuel system. Some of the evidence seems to point towards ill effects of fuel containing ethanol on engines. I'll leave that to the experts.

I'm not sure ethanol is a huge concern in non aviation settings, but most vehicles now have systems which adjust ignition timing based on data gathered by electronic systems- our aircraft do not, and have fixed magneto timing. There seems to be lots of evidence for fuel containing ethanol to provide lower mpg versus old school "straight" unleaded too. I wonder how that translates in an aviation setting where we push our engines pretty hard, they are air cooled, are expensive, and we demand utmost reliability and full power in many situations from them.

I would NOT use auto fuel containing ethanol in any aircraft system where I was not 100% sure that the entire engine and airframe system could handle it with no ill effects. How does one determine that on their own? My answer is that legally in a non-experimental aircraft- they can't. I'll stick to that.