(<reads>)

In practice, this seems to break down at a specific point: this can be outlined, for instance, with the hypothetical stipulation "...and possesses the technology or similar power to cross universe boundaries and appear visible before me in my room, and will do so in exactly ten seconds.".

As with the fallacy of a certain ontological argument, the imagination/definition of something does not make it existential, and even if a certain concept contains no apparent inherent logical impossibilities that still does not mean that there could/would exist a universe in which it could come to pass.

'All possible worlds' does not mean 'All imaginable worlds'. 'All possible people' does not mean 'All imaginable people'. Past a certain threshold of specificity, one goes from {general types of people who exist almost everywhere, universally speaking} to {specific types of people who only exist in the imaginations of people like you who exist almost everwhere, universally speaking}.

(As a general principle, for instance/incidentally, causality still needs to apply.)

Edit: <amuses self by imagining an immortal near-omniscient mind which would still engage in conversation with the exact replies that a mortal less-than-100-years human writing in a notebook would come up with, rather than say something more insightful/helpful or influenced by more experience/knowledge>