Hmmm, I can see it now - instead of just dipping tanks on diesels for off-road fuel, they'll be checking gas cars as well for ethanol.
My first guess is the EPA doesn't give a hirsute rodents posterior about whether or not it's workable right now - as in the past, they'll simply throw out a new regulation, and then tell industry and the country to "deal with it and make it work".
IMHO, this isn't about air quality at all - this is simply Part B of the government propping up the ethanol industry. Part A was Congress enacting the 45 cent/gallon tax credit for blending E10. And now the EPA is kicking in with Part B to force everyone to buy this crap.
Worst case scenario - the EPA just says screw it and forces E15 down everyone's throats regardless of the MY of your vehicle, forcing everyone with a non-compatible vehicle to either retrofit or repair it, or buy one that is.