enough magic into it to overwhelm anything a first-year student could produce

That wouldn’t be quite enough; Harry could have got someone else to help, though indeed it is unlikely.

assuming that Finite or Finite Incantatem can affect artifacts, which we don't know to be true

You’re quite right about this. Presumably there’s some sort of “stabilizing” element for artifacts and spells that are meant to be (semi)permanent, precisely to avoid accidental Finite. (Not necessarily brute-force resistance spells, it could be just a safety thing to make sure you really want that cancelled.)