[Decimus]

So just to be clear...

Argument 1: Whether Robhol used the word 'imperative' correctly or incorrectly.

Let's drop it for now until Robhol gets back and clarifies his position.

Argument 2: Whether 「お-なさい」 is a valid form of the imperative mood. (this is a purely grammatical question.)

Agreement : 「お/ご-なさい」 is a valid form of the imperative mood.

Argument 3: Whether 「お/ご-なさい」 is an obsolete form of the imperative mood.

You intend these as three separate arguments, and you do not think that you are moving the goalposts from Argument 1 to Argument 3.

Is that the case?

Or substitute whatever word you think is more appropriate in place of "obsolete".