What impromptu moment? If you're talking about "her microphone" it was not impromptu. It was clearly planned as her little " oh look it worked" smirk gave away when people cheered it. It doesn't mean it didn't work, but it was not impromptu.
I think she has a good chance to get the nomination. The only threats I see remaining after the first two nights are Mayor Pete, Bernie, Booker, maybe Warren who is falling IMO, and maybe Castro who is rising IMO. The rest are toast with Gillibrand having a minor chance to fill a void if someone slips up badly soon.
If/when she gets the nomination I think she is the only one with a chance but she will far from mop the floor with Trump. Whoever delivers the message, the commie crap doesn't play well.