Is a divorced woman required to inform her ex-husband of her marriage and subsequent divorce from another man, knowing that he will not accept it and it may cause problems?

You are right to refuse to return to your first husband who divorced you with three pronouncements and denies it. A woman must abstain from her husband in such a situation. There is no blame if you return to him after marrying someone else in a legitimate marriage, without the intention of making yourself permissible for him [tahlil], and after your waiting period (iddah) has ended. One should weigh the expected benefit of returning to him against not doing so. As for informing him of your previous marriage and divorce, the jurists have not mentioned that this is a defect that must be disclosed. If the marriage fulfills the conditions, it is valid.