Can the sins of a deceased person be expiated, especially if they committed a sin, by performing Umrah on their behalf? And can a wife's prayer for her husband lead to Allah's forgiveness of his sins?

What you did by supplicating against your husband unjustly and depriving him of yourself are two grave sins. Since it is too late to seek forgiveness from the one wronged, you must repent to Allah, the Almighty, and be sincere in your supplication to Him, for the door of repentance is open. If the husband has forgiven you for the nushooz (disobedience/rebellion), then that is beneficial because nushooz is a right of the husband that is waived by his waiving it. As for performing Umrah and seeking forgiveness for him, the reward for that will reach him and benefit him, God willing, and it is proof of the sincerity of your repentance. Every good deed performed by a living person, and whose reward is dedicated to a deceased Muslim, will benefit the deceased and that reward will reach them.