Is it obligatory for a man to inform his fiancée about his struggle with infertility and about his previous wife's request for divorce due to it, or is it permissible for him to postpone that until after marriage?

There is no obligation to inform the wife about not being able to conceive, and there is no sin in not informing her. Informing her is better, to preempt potential disputes. It is advisable to continue with treatment, supplication (du'a), and seeking forgiveness (istighfar).