There's certainly a chance that he will lose the case. But to me it seems unlikely just based on what needs to be proven for a defamation suit in the US.
1) You need to be able to prove that a statement that was purported as fact was indeed false.
This is absolutely the case here. The original accusatory statement was made on twitter, and all the contradictory statement were also made on twitter. It will be difficult for a defense attorney to claim that while the original statement was meant to "inform" public opinion (especially with her also being thankful for Avellones firing on twitter), that the other posts were just inane ramblings that were not meant as fact. It is very obvious, thanks to the extensive post history of Karissa, that she is not only a sociopath, but also lied about a variety of statements after the fact. A competent defense attorney could claim that recollection of events back then was clouded due to alcohol and that her later statements are therefore to be considered more accurate, but that can easily be disputed by witness testimony confirming most of her original postings. It would be weird if she remembered and tweeted about everything accurately, EXCEPT how she felt about the evening with Chris.
2) You need evidence of said person communicating or publicizing the false statement from 1) to a third party.
This one is pretty obvious. She did so, not only to "inform" people about Chris, but also thanked people on numerous occasions that he did in fact get fired.
3) You need to prove that the falsified statements were due to negligence or malice.
This is where you have to worry about whether Chris actually is a public figure or not. If he is, then negligence is not enough. Proving malice is difficult, but not impossible, especially with how off the rails Karissa is on twitter. You just know that she's been even worse than that in private, and it seems to me, based on Chris medium post, that he still has access to the correspondence from when he and Jackie were dating. She clearly threatened to do 'something' and as such it may not be as difficult to prove malicious intent. Still, this one is the part that might trip Chris and his lawyers up, unless they have a smoking gun.
4) You need to prove that there were damages or harm caused by those statements.
You would literally just need a signed affidavit by one of the companies that fired Chris, that it was due to the allegations in 1) that he was let go. Even without that, the correlation is pretty clear.
Regardless of whether they can actually prove malice or not, it is likely that any sane judge will realize that the situation at hand is more than just Chris trying to get some money out of this. The defenses case essentially rests on claiming that Karissa did not understand the repercussions of her actions and how her false claims would affect Chris. But she's been acting like a rabid animal for years on twitter. Remember that she deleted 60.000 tweets in 8years. That is ~20tweets a day. Every day. That is an INSANE amount of tweets to comb through, and plenty of evidence of her spouting inane and dumb shit will be found in these - sidenote, I want to pity whatever paralegal was asked by the actual counsel to go through each and everyone of these, using slow as fuck archive websites, to find the most incriminating ones and put them into the suit. David Cernats compiled list he used in the initial video must've been a huge help for this purpose alone). Chris most likely has access to private correspondence, witness testimony of people supporting his side as well as the big unknown - Jackie. This suit becomes a lot harder to lose if Chris initial apologetic stance made her more sympathetic to his side. If it was in fact a concerted attack on Chris person, tripping up one or two of the other "accusers" and showing private correspondence and turning just one of them might unravel the entire thing.
But that is treading into "what if" and conspiracy territory and I'd rather not do that. Chris is right in asking victims to speak up more in general, by the way, as the more information and details are being presented, the more likely it is for people to believe them. A short "this dude raped me" shouldn't cut it, and giving details for people to weigh in and support (or speak up against) the statements is the only way the actual truth can come out.
tl;dr: If a decent Judge gets this case, it should be fairly easy to prove all of the points needed for a defamation/libel case to be made. If he doesn't, even a liberal left-leaning judge with a hateboner for CIS-gendered white males will have a hard time supporting someone who is as off the rails sociopathic as Karissa was on twitter. It doesn't matter how she acts in court, Chris attorneys have 8 years worth of 20 tweets a day to characterize Karissa. The way I see it, a defense attorney will have a hard time refuting any of this except for the "malicious intent" and we simply don't know enough about their private correspondence or what witnesses Chris referred to in his medium blogpost to understand whether he has a hard time ahead of him, or went into this knowing full well that Karissa is fucked - and not in the way she clearly wanted to be while he was shtupping her friend.
Edit: Witnesses in this case does not mean that they would have to come out and straightup say "he did not assault her". Witnesses supporting his side could be something as simple as a hotel employee confirming that the room Karissa and her roommate were sleeping in only has 1 bed (making it easy to refute some of her earlier statements about him being inside her room without the roommate noticing), or people who were around during the other incident that could simply say "I did not see Chris Avellone touching her indecently". The goal of Chris lawyers is to point Chris as an upstanding citizen who has no previous record of misconduct, defusing the claims that were obviously false, potentially getting one of these witnesses on his side in order to cut a deal with them, while portraying Karissa as a vindictive jealous sociopath. As said before, if witnesses can claim that most of the statements during the 2 nights in question were accurate, there is no reason to believe that the ones about Chris are not. Proving that is easier than what happened between the two of them in private. Thankfully she essentially logged every minute of her private life in detail on twitter. Fucking retarded.
Edit 2: Most of the issues these twitter "lawyers" have with the suit, are due to the fact that Chris and his lawyers are using the legal system as intended. They filed the suit in a thorough manner (citing >100 defendants in order to not bring up additional ones mid-hearing), citing ~90 pages of evidence in order for opposing counsel to be able to properly prepare for it, filing it where Chris actually resides, and not trying to get around the need to prove malice by straight up claiming that Chris is a public figure. Not doing any of these would just drag out the case, potentially create issues for the defense and while any of those are technically legal ways of stacking the deck in Chris favor, they're so blatantly obvious that most Judges will build a bias against the legal team trying to pull these kinda tricks unless they have actual reason to do so. The suit is so "in your face" straightforward, that it leaves the defense with very little room to file for dismissal, which is exactly what Chris wants/needs. Based on Karissas tweeting history, having this thing land in actual court is not something her attorney would hope for. It also happens to be what most of these 30y something lawyers would want, as they grew up in the tv-show school of lawyering where you never wanna go to court unless you absolutely have to, instead trying to find ways to dismiss cases or come to agreements outside of court to avoid a public hearing. They can't fathom the idea that someone would actually WANT to have a judge preside over their claim.
