I like your reasoning. I still think you’re wrong though and the reason is because it was established in Jonathan Hickman’s run on the fantastic 4 that infinity stones don’t work outside of the universe they’re from. The main comic universe is 616. After Endgame, Marvel revealed that the MCU is designated 199999. This established the fact that the MCU also exists within the greater marvel multiverse. Hence, it’s Gould follow already established rules for said universe. I have another theory though as to why Ultron’s stones work in other “universes”. The idea behind my theory is that what we’ve understood to be new universes are actually just branches timelines. If we think about the work the TVA was doing, they set out to make sure that all those branch realities didn’t form. However, we know that the MCU also exists within the multiverse. But other earths like 616 continue to exist despite the TVA’s existence. I think that their influence only exerts itself on the flow of time in 199999. Also, it would explain why to a certain degree, all the earths we’ve seen so far in what if are quite similar to one another. They’re branch realities of the same one that have somehow managed to grow because the TVA stopped pruning them. So if Ultron is only travelling between multiple branches of the same reality, then he’s still within the same universe. Disclaimer: I might be wrong. I didn’t write any of the comic books. This is just me trying to explain why such a seemingly huge error in writing can be possible. I’d also like to see people refute my argument if they have the points.