Nice theory. The only thing I would ask you to consider as well is Izanagi. This could have also been used to avoid some of the encounters and we know Obito has a store of Sharingan. I think Kishi has explored the topic and I agree that Kamui has some ability to manipulate time. Here we have Obito arriving at the battlefield where Sasuke and Itachi fought and Obito takes Zetsu to task.
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Now Zetsu may have just been sarcastic but traveling in Kamui was much faster than Mayfly. Also, the pocket dimension consists of cubes and a tesseract (hypercube) would be an example of a fourth dimensional object, so yes spacetime is part of Kamui.