I always forget about Sinema, which is something I need to stop doing because, as you say, she's probably the true believer.

I don't think we can be too charitable towards Manchin considering his other stances. That said, you raise a good point: changing the filibuster doesn't really change his position; he's still an essential vote to pass any legislation in a tied senate, so he benefits from either outcome. I suppose, all other things being equal, it makes sense for him to nominally oppose ending the filibuster for the sake of his base.