I think I've come to the conclusion before that it's causative, and [us part] is a small clause — otherwise the case on 'us' is mysterious

I might be confused about the issue here, but even if "do" is a verb/tense/auxiliary thing wouldn't it still be "us"--"till death does part us"?

maybe! im also completely comfortable with us being accusative bc it's just straightforwardly an object of part, a verb, and do is there for like emphasis in the way we get do-support for emphasis sometimes