Buttoning of 3-piece SB jacket

[HappyStroller]

I was taught to button up the single breasted jacket of a 2-piece suit when not seated.

But are there other rules regarding the buttoning of the sngle breasted jacket?

Must the jacket be buttoned when not seated if a vest is worn?

Should the last button, if there is more than one, of this jacket be left unbuttoned when not seated?

Is there any rule to keep the jacket buttoned up when seated, e.g., when having an audience with a king or head of state?

If the jacket was a Morning Coat or a Stroller, would the rules be the same?