“Who knows”, “Who’s to say”, and “God knows”: How are these three expressions nuanced in meaning?

Factual Questions
1

I was wondering if I can use any of these three expressions in the following sentence – possibly with slightly different meanings? Many thanks.
Man: You know, this marks the fifth time I’ve found you in the midst of trouble. If I didn’t know any better, I’d say you’re a lodestone for it!

Who knows what grand conspiracy you’ll stumble upon next!?
{ vs “Who’s to say / Who can say what grand conspiracy you’ll stumble upon next!?” }

{ vs “God knows what grand conspiracy you’ll stumble upon next!?” }

2

To me, as a native English English speaker, the difference is more style than meaning. “God knows” can offend some anti-religious people so is often avoided these days.

There are any number of similar phrases from the rude - “Fuck knows” to the mundane - “I don’t know”.

3

There are only fine differences, and it would depend on the speaker, I think. To me, “Who’s to say” might imply a focus on actual possibilities, whereas both "Who knows " and “God knows” (as a slightly more emphatic form) imply a focus more on the general outrageousness/incomprehensibility of possible events. The former might anticipate expect some sort of continued discussion of the event, the latter is more a rhetorical indication of surprise/anger that may not lead anywhere much.

4

The biggest difference I can see is that “who’s to say?” can have implications of “who am I to judge?”, on a question of morality or philosophy. A statement like “Who knows if polygamy is even moral?” implies that there is an absolute morality relevant to polygamy, and we just don’t know what it is. By contrast, “Who’s to say if polygamy is even moral?” leaves open the possibility that there isn’t even an absolute morality to be known.

5

To me, “God knows” contains at least a hint of one of the following implications:

  1. He might know, but nobody else does, or
  2. I don’t know and I don’t care.
    depending on the tone when it is said.
6

Who knows – Said with a shrug and a “meh” expression. “Why doesn’t management give us the right tools for this job?” the new guy asked. “Eh, who knows,” his coworker replied.

Who’s to say – Said with raised eyebrows and a thoughtful expression about a topic that is either debatable or unprovable. “Who’s to say if we’re the only inhabited planet in the universe?”

God knows – Said in an exasperated tone with a roll of the eyes and/or shaking of the head. Not a question but a shortened version of “Only God knows…” “God knows what Lucinda was thinking when she bought that pig.”