I'm no lawyer, but I'm missing something here. why would the constitutional language address the validity or illegitimacy of a former officeholder candidate guilty of sedition and not provide language about that officeholder's immunity from wrongdoing. because there is none perhaps?
I don't think Article 3(14) necessarily refers to former officeholders, but rather sets a qualification for future officeholders. Supposedly immunity is granted to acts committed in furtherance of presidential duties, though such duties do not include campaigning for the next term.