Could such professors who signed the letter be at risk of losing their tenure, because the letter could now constitute a form of retaliation and defamation of the victim (the male student)?
Most of the letter is not defamatory, and consists primarily of a character reference for Prof Ronell. However, there are a few parts in the first paragraph (the highlighted parts) which claim that the complaint is malicious, and this could definitely constitute a defamatory statement against the male complainant in the case. It would be open to the complainant to sue the professors who wrote the letter, and they would have standard defences available to them. This includes a "defence of truth" which means that they would be given an opportunity to prove their assertion that the claim was in fact malicious (and if they could prove this on the balance of probabilities it would constitute a successful defence). Given that the university has found that Prof Ronell did commit the conduct that was the subject of the complaint, it would be hard to prove that the complaint was malicious.
Note that in a defamation claim there would also be an issue about publication of the letter. The letter evidently was not intended as a public document, and was only sent to potential signatories and the two NYU staff to whom it was addressed. It was made public by people other than the signatories, due to its newsworthy character. If the signatories were sued for defamation they would probably try to defend the claim on the basis that they had not intended any publication of the letter. (Some of the signatories, such as Jizek, have made subsequent public remarks on the case.) In defamation actions you can still get in trouble for non-published speech that is sent only to a smaller group of people, so there would probably be some argument on this.
As to retaliation, that is a bit trickier. The NYU Policy on discrimination and harassment can be found here, and it states that it applies to all employees of the university. In regard to retaliation against a complainant, the policy states the following (emphasis added):
The University will not in any way retaliate against an individual who reports a perceived violation of this Policy, participates in any investigation, or otherwise opposes perceived discrimination, harassment (including sexual/gender harassment), or retaliation, including as a witness. It will also not retaliate against anyone associated with the individual who engages in such protected conduct, such as a family member.
NYU further will not tolerate retaliation by any employee. Retaliation against anyone who complains of, testifies in, or assists in an investigation or proceeding involving discrimination, harassment (including sexual/gender harassment), sexual assault, or retaliation is a serious violation of this Policy, as well as federal, state, and local law. Anyone who believes they have been subjected to retaliation should report the matter immediately according to the same procedure provided in this Policy for making complaints of discrimination, harassment (including sexual/gender harassment), or sexual assault. Any person found to have retaliated against another individual will be subject to the same disciplinary action provided under this Policy for other violations.
Based on this policy, it would seem to me that there is a strong prima facie case for retaliation proceedings against any of the NYU professors who signed the letter. It explicitly accuses the complainant of maliciousness, which would imply that the complaint is false.
Note that the complainant, Nimrod Reitmann, has filed a lawsuit against Ronell and NYU in relation to this matter (see here). The article reporting the lawsuit does not mention any action for retaliation relating to the letter of support, but it would seem to me that this would be a possible cause-of-action that could be added.
The letter of support for Avital Ronell
The letter of support in this case can be found at the blog of Philosophy Professor Brian Leiter here. Here is the first paragraph of the text with highlights of parts that are possibly defamatory to the victim in the case (emphasis added):
We write as long-term colleagues of Professor Avital Ronell who has
been under investigation by the Title IX offices at New York
University. Although we have no access to the confidential dossier, we
have all worked for many years in close proximity to Professor Ronell
and accumulated collectively years of experience to support our view
of her capacity as teacher and a scholar, but also as someone who has
served as Chair of both the Departments of German and Comparative
Literature at New York University. We have all seen her relationship
with students, and some of us know the individual who has waged this
malicious campaign against her. We wish to communicate first in the
clearest terms our profound an enduring admiration for Professor
Ronell whose mentorship of students has been no less than remarkable
over many years. We deplore the damage that this legal proceeding
causes her, and seek to register in clear terms our objection to any
judgment against her. We hold that the allegations against her do not
constitute actual evidence, but rather support the view that malicious
intention has animated and sustained this legal nightmare.