Is the phrase "Just as we entered with kindness, so shall we depart with kindness" considered one of the allusions to divorce, and does divorce take place by it, knowing that a clear divorce already occurred once?

A metonymy for divorce is any word that indicates separation but is not explicit about it, and it does not constitute a divorce unless there is an intention of divorce. Your statement, "Just as we entered with kindness, we shall leave with kindness," and your mother-in-law's reply, "And I will only bring what I have brought," could potentially be a metonymy for divorce if it is understood عرفًا (customarily) to mean separation. However, it is not considered a divorce if you did not intend to execute and enact the divorce, but rather wished to know your mother-in-law's response. Spouses should overcome disagreements through understanding and tolerance, and divorce is the last resort.