Shouldn't we be concerned with the number of sentients that our Earth-derived lineage will contain, not the total number of sentients that will ever exist anywhere?

I thought it would end up working out the same. I just added that part, and I was wrong. It's still somewhat close.

How does "believing that Xs matter less" imply "it is less likely that you are an X"?

It's not proof, but it's evidence. I guess it's pretty weak, so I deleted that part.

As a completely unrelated justification for that, if you ran a human brain slower, you'd be less likely to be that person at a given time. For instance, if it was half as fast, you'd be half as likely to be it at 5:34, since, to it, 5:34 only lasts 30 seconds. Animals are less intelligent than humans, so they might act similar to a slowed human brain.