State's comments in notice and comment period were not specifically contrary to its positions in the present litigation; it therefore has standing to raise the challenge.
Plain meaning of the statute presents insufficient ambiguity to permit a challenge arising from context.
Evidence of a state's extrinsic noncompliance is insufficient basis to hold that the agency's decision that the state's lack of enforcement actions indicated compliance with the plan was arbitrary or capricious.
As the act is an exercise in cooperative federalism, agency was within rights in accepting state's assertion of compliance.
https://www.cadc.uscourts.gov/internet/opinions.nsf/A1B19B3B5B48B063852582C600523288/$file/16-1230.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.