When searching for a shidduch, what details or personal information may one disclose about a former wife to a shadchan, potential wife or anybody else relating to the failure of the previous marriage? What is categorized as lashon harah?
You can disclose all relevant information, even if this casts the former wife in a negative light.
The Chafetz Chaim actually writes that it might be permitted to speak negatively about others for the purpose of clearing one’s name. This is certainly true in the area of a Shidduch where the purpose is not only to clear one’s name but also (and primarily) to seek to further the cause of the Shidduch.
Therefore, you can speak liberally insofar as there is some relevancy to the proposed Shidduch.
Best wishes and good luck.