While I agree with the last part, I'm not so sure of the first. In the extraordinary event that a state party defied the national party and circulated nominating petitions for someone other than the national nominee, I wonder what laws would govern it? To provide injunctive relief in such a situation would mean that a judge would have to take perfectly legitimate citizen petitions and throw them out based on.....what? That's some pretty extraordinary interference in an election process.
However, this is all hypothetical. It isn't going to happen. For purely practical reasons, even the places where Trump is hated no state party would actually go that far. I'm just wondering if it is a legal option to even do so.
On a related note, if Trump does drop out, which is also incredibly unlikely but not absolutely outside of the realm of possibility, do they have the right to replace him even then? I know that some states have rules about replacing names on ballots for the partisan slot. If the nominee has won the primary, his name goes in. There's no provision for the nominee dropping out voluntarily. I don't know how that works in the case where the process for naming a candidate in a state includes things that happen outside of a state, including at a private gathering such as a nominating convention.