Is it considered a lie and a breach of trust to conceal a previous, Sharia-compliant interaction with men, especially given that an oath was sworn regarding it, and considering the fiancé's repeated questioning, suspicion, and jealousy? And is it obligatory to disclose this, despite the possibility of separation?

Neither spouse should question the other about their forbidden past, and trust should prevail between them. Jealousy is praiseworthy if it arises from suspicion, but blameworthy if there is no cause for suspicion. Lying is forbidden, but Islamic law has permitted spouses to lie to each other for the sake of maintaining the marital relationship. However, it is preferable to use allusions (al-ma'areedh). If you lied to him before marriage without using allusions, you must repent. It is not permissible to swear falsely except out of necessity. One should be careful to avoid situations that arouse suspicion or temptation.