In re Neagle (while, unfortunately, it does not state as clear of a rule as Horiuchi on the standard that should be applied) conducts an expansive facts-based analysis on the question of whether, in fact, the acts performed were done in in the performance of his lawful federal duties (if anything, the implicit standard seems less generous to the federal officer than Horiuchi’s explicit rule, which would allow Supremacy Clause immunity if the agent had an actual and objectively reasonable belief that he acted within his lawful duties, even if, in fact, he did not.)

But, yeah, any state prosecutions (likely especially the first) is going to (1) get removed to federal court, and (2) go through a wringer of federal litigation, likely reaching the Supreme Court, over Supremacy Clause immunity before much substantive happens on anything else.

OTOH, the federal duty at issue in in re Neagle was literally protecting the life of a Supreme Court justice riding circuit, as much as the present Court may have a pro-Trump bias, I wouldn't count on it being as strong of a bias as it had in Neagle.