I agree that the far future deserves more attention than most people give it. :)

The basement universe might be much bigger than our apparent universe, but even if so, that would allow for lots of short-term sims, so it’s not obvious that the ratio of short-term vs. long-term sims would change. What really matters is what fraction of sims run for how long. If there are only a few sims that are very long, then far-future stuff matters even if we’re in a sim. If most sims are pretty short, then there’s a lot more sentience that can be helped by short-term actions. Also, long-term sims might contain short-term ones (a sim within a sim).

I don’t think a singleton would mind being simulated in general, because that only increases the number of copies of it that exist. Maybe your idea would make sense relative to evidential decision theory or something, but I haven’t thought about it enough. :)