(1) Can I replace the highlighted expression with 'Mind your own business' or 'That's my business'? -- You need to recast: It's none of your business how much I weigh/earn/etc. That's my business— how much I weigh/earn/etc.
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Mister MicawberAre there any other expressions/idioms to that effect?-- How many do you need?!The more the merrier