In my opinion, these are all correct and natural. "To do so" has no tense. I'd say it's only the conditional quality that's essential.
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zazzexHello, Falks,This one is strange, though. Leave out the 'to do so' here, or alternatively put a comma after 'to do so' (perhaps even dashes).
1. When it becomes necssary to do so to take down the government, we will join you.
Is the above correct to have "to do so" refer to " to take down" following it?
AvangiI read "to take down the government" as parenthetical, or an appositive, or something like that.Yes, that's why I suggested a comma or dash to make it so. Appositives require commas. (it's an infinitive appositive phrase modifying 'so', as far as I know...)
It's an alternative to "to do so."