Here's a simple and direct argument for my version of verificationism.
Note that the argument uses ontological terms that are meaningless on my views. It functions as a reductio - either one must accept the conclusion, or accept that some of the premises are meaningless, which amounts to the same thing.
Premise 1: The level IV multiverse is possible.
Premise 2: If the level IV multiverse is possible, then we cannot know that we are not in it.
Lemma 1: We cannot know that we are not in the level IV multiverse.
Premise 3: If we are in the level IV multiverse, then ontological claims about our world are meaningless, because we simultaneously exist in worlds where they are true and worlds where they are not true.
Lemma 2: If we can know that ontological claims are meaningful, then we can know we're not in the level IV multiverse.
Conclusion: We cannot know that ontological claims about our world are meaningful.
Edited to add two lemmas. Premises and conclusion unchanged.