1. Why should MSNBC correct Avenatti? He said what he said. People go on TV all the time and lie their asses off, er, that is, give their opinions about the news of the day. The network didn't report what he said as news; they put him on the air and he said what he wanted to say. Just like every other talking head they put on every day. That's the difference between opinion chat shows and straight news, or it's supposed to be.
2. Of course the Trump people are feeding bad info to the media to try to discredit them. It's the Dan Rather model - put a fake memo or, in this case, a juicy nugget of info that's hard to verify immediately, that is false by design, and if they swallow the bait you use it to discredit the whole story and the network and the entire mainstream media, if you can. It depends on having a willing echo chamber to shout about the fakery of the media, but of course they have that in spades. The nets should be aware of this tactic - I'm sure they are - but obviously NBC's greed for the scoop overrode their judgement yet again. Their own fault; they need to be better.