Validating existence should already be trivial: virtually all journal already have publicly-available indices which contain at least author information and an abstract. Combine that with DOI citations and you're basically done - even with closed-access journals.

Checking the content is of course a lot more difficult, but that doesn't magically become trivial with open-access journals: you still need to read and interpret what is being said in the paper and compare it to the claims being made in the citation. Granted, these days you could use AI for a first pass, but it's still going to be incredibly tedious work.