how best to say 'what's mine is mine'? Will 'sa ki pou mwen se pou mwen' work? And can I do the same with 'what's yours is yours'?

Yes.  It'll work.
You can also say:  Pa'm se pa'm

The same for "What's yours is yours", including the contracted version, you can also say:
Sa ki pou ou se pou ou.
Sa'k pa'w se pa'w.
Pa'w se pa'w.

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