If I follow the definitions of the OP’s post, then I’m introvert and not ambivert, since I only recharge my energy when I’m alone. I am however, a bit of a weird introvert (at least according to online personality tests and so on). Since, besides the whole “recharging energy” thing, I tend to fulfill a lot (if not most) of the traits assigned to “extroverts”.
Regarding your scenarios:
(Now this has actually happened far more than I would’ve liked to, in my life ^^’ ). It depends a little on the situation, but in general, I’ve gotten to a point in my life (26y.o.) that I’ll just own up to the fact that I can’t remember him, suffer from embarrasment and then figure out who he is. If it had happened while I was a teenager I would more likely try to play along, while desperately trying to figure out who he is.
If I know the people: I would probably feel a little left out, or like a third (or whatever uneven numbered) wheel, especially if we’re out together. I’ll either try to get involved in the conversation, find some other people I know to talk with (If there are other people I know at the event/place) or leave and do my own thing for a little while.
If I don’t know the people but they know each other: I would feel a little weird, since I would literally be a stranger in the middle of a group. But not really caring, and keep doing my own thing.
If none of us know each other: I would probably feel a little awkward and out of place, either trying to engage in conversation (with more difficulty than if I know the people), or do my own thing.
tl;dr: Depends a lot on my mood and the interpersonal relations between myself and the other people in the group.
(I image I haven’t experienced the colleague behaving like this before) I would firstly be shocked by how much of a ahole the person is, and then getting f*ing pissed at them on behalf of my friend…I can’t really say how I would react. I would probably confront them by asking them why they think they are being funny.
If i knew the colleague normally behaves in this way I would at best begrudgingly interact with them, at worst looking for another job if it was possible. I wouldn’t want to work with a person like that.