I feel like if you are playing into the Factory Reset 3PO, it has to be a line that is played straight, but is funny because it’s naive.

Trooper: What’s your operating number?

C-3PO: I am here to negotiate the release of a political prisoner named Chewie. (the trooper would start talking over him)

Trooper: Negotiate- what?

Then when Rey comes in guns blazing, you could even replace 3PO’s next line with:

C-3PO: Oh! That was rather aggressive.

A dumb reference, but maybe some deep recess of 3PO’s memory unit made that connection. A shorter version of that line could just be:

3PO: I am here to negotiate for Chewie.

And you could keep the trooper’s “The why are you- what?” like the same, but I like the idea of 3PO saying “Chewie” being why the trooper asks “what?”

Also, I think if you kept playing with this idea, you could change 3PO’s “Happily” line when they first land in the hangar with something more formal and straightforward. Or, just cut it out. I think at this point it makes sense not really refer to his emotional state this early after his memory wipe. It could be funny if we heard him start saying something like, “Excuse me, but this is very much against protocol…”
That would play pretty funny I think, especially coming off his previous line, “And I will be your protocol droid!”