Not really keeping up with this thread, but here’s a random comment.
For me, after getting the full Evil Genius argument, Descartes' Epistemology (Stanford Encyclopedia of Philosophy/Winter 2003 Edition), and realizing he ends with a “first cause” argument for God, I had to think this out for myself. This was probably about the time someone was on this forum make a “zombie” argument, Zombies (Stanford Encyclopedia of Philosophy/Winter 2003 Edition), which is taking the brain in a vat to the extreme that you are the only brain, and everyone else is fake.
Anyway. In the end, we only have the tools that we have, no matter what is “real”. The tools include concepts, like accepting that we can’t be 100% sure about what we know. We can calculate probabilities, but let’s say Descartes’ experiment is probable, and we’ve been wrong about physics for the last 500 years. It doesn’t change the tools we have to determine what the evil genius is and maybe even how to escape its prison. Whether we are hooked up to a Matrix, or beings on a planet surrounded by other life, the way we figure it out is the same.