Is the suitor entitled not to disclose the reason for his previous divorce, on the pretext of not slandering his ex-wife, or is he obligated to disclose it for reassurance?

It is impermissible to mention a Muslim in a way they dislike. Therefore, it is impermissible for a man to speak about his divorced wife in a manner she dislikes, nor for you to inquire about the reason for her divorce, because that constitutes backbiting (ghībah). Such talk will not benefit you in discerning the suitor's character, because in disputes, each party views themselves as in the right. The Islamic methodology for marriage is to choose someone of good character and religious commitment, and to ascertain this by asking people known for truthfulness and sincere advice. One should also perform Istikhara (prayer for guidance) before undertaking any matter. If your heart is reassured by his reputation and seriousness, then rely upon Allah.