Merlin created the Interdict because he believed, based on prophecy, that this would prevent the otherwise inevitable end of the world and its magic.

He hoped it would, but he didn't live to ask the remaining seers if it actually worked.

If resolved!Harry is "the end of the world", as per Trelawney's prophecy, then whatever he is going to do must therefore involve bypassing the Interdict of Merlin.

This doesn't really make sense, or is irrelevant, or is sort of a tautology, or something. The Interdict of Merlin is not a magical universe-saving spell. If it were, as you sort of imply, then you would basically be saying "If Harry can destroy the universe, then it follows that he will not not be able to destroy the universe". But the Interdict is not that, nor does it limit what magic a person can use; it simply limits the transfer of the most powerful magics to only occur between two living minds. Merlin hoped that would be enough to save the universe because he counted on magical knowledge waning permanently.

The only way we presently know to do that is Salazar's basilisk-transmitted lore

Patently and obviously false. We've known since Chapter 77 that Nicolas Flamel has a whole bunch of knowledge he might someday share, and the whole point of Harry James Potter-Evans-Verres since Chapter 28 is that he can figure out magic that's long-lost or never-known. And the Interdict imposes no limitation on figuring out powerful magic on your own.