There is no E15 "mandate", but truth in headlines was lost a long long time ago. Once again, the government has NOT mandated E15. The only thing they have done is to approve a request by some fat cats who want to sell that lovely stuff to us and line their pockets. The answer to this doesn't lie with the courts, it would be for Congress to pass a law that would actually "mandate" what can and cannot be sold. Don't hold your breath however, the corn lobby and the ethanol producers lobby have a lot of clout ($).
As far as I'm concerned, if they want to sell properly labeled E15 alongside E10 then it's not a problem. If they want to allow stations to eliminate E10 and only sell E15, however, that is not acceptable. There is still way too much equipment in use that is not compatible with even E10, much less E15. It's not fair to make all that equipment obsolete or require expensive modifications to enable it to use the fuel. Of course the folks selling all that equipment might not agree, since they will get to sell a ton of new outboard motors, lawn equipment, motorcycles, cars, trucks, generators, etc. to replace the stuff that still works fine now but won't if the fuel they depend on disappears. Kind of like the people with old cars that depended on lead in the fuel to lubricate valves and seats and who were forced to replace those parts or find lead additives at additional cost.
Jerry