To even get a serious case in a state court, some state’s secretary of state would have to disqualify him from the ballot. Even then, a court might doge the issue by calling it a political question.
I agree with that, although dodging the question puts the court in a position of tolerating a ballot with potentially ineligible candidates.
The RNC can disqualify him without engaging a court. That would be an issue before the nominating convention, and be independent of election contests.