What is the ruling on an existing marriage and its consequent provisions—such as spousal cohabitation—if the wife committed what might be considered an act of disbelief (kufr) or apostasy (riddah) either before or after the marriage contract or consummation, especially if she does not recall the exact timing of that? And is she obliged to inform her husband about it?

The questioner should repent, increase seeking forgiveness, and perform good deeds. There is no basis for ruling apostasy after this long period (four years). A word that could imply disbelief or something else does not lead to a ruling of apostasy against its utterer, even if the probability of disbelief is higher. The two mentioned phrases: "I wish this verse was not in the Quran" and "May God curse your mother's religion" are not explicit in disbelief, as the objection in the first might be directed at those who use the verse as evidence, not at the verse itself, and the insult in the second might be directed at the mother or her behavior, not at her religion.