A Sandbox should work (use "Shared users"), but please ensure you have a backup of your real drive before you do this, in case you have any issues.
I honestly don't know what Time Machine is going to do. It may delete your old backups if you boot from some other drive. I'd probably leave them completely disconnected while you check compatibility.
Not sure what "then what" you're looking for in #3...
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--Dave Nanian
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