How should I act regarding my wife's complaint that my father tried to kiss her against her will in my absence, knowing that my father is of sound mind and a virtuous educator?

The father's harassment of his son's wife is a grave evil and a sign of weak faith. The presumption is that the father is innocent until proven otherwise, and you are obligated to honor him and treat him well. If the wife perceives any suspicious behavior from the father, she should advise him and remind him of Allah. If he does not desist, she should threaten to inform the son. Otherwise, she should treat him as a stranger, not allow him to be alone with her, and not remove her hijab in his presence. If she lives with him, she should reside in an independent section with its own facilities. If it's possible for her to stay with her own family, that would be better. We advise you to strive to reunite with your wife.