Is a divorce pronounced by a husband, contingent upon his wife's infidelity—which he interprets as a relationship, correspondence, or phone calls with another person, or even love in the heart—valid, especially if the wife confesses her love for another person, knowing that the husband was deceptive in his intention to divorce with these words?

As long as you have made your wife's divorce conditional upon a verbal or messaging relationship between her and a non-mahram, and you have not initiated another conditional divorce, then her divorce does not occur merely by her loving a non-mahram, whether you were truthful or untruthful in your statement to your wife. This is because you do not have the power to change your intention regarding what you made the divorce conditional upon. Therefore, her divorce did not occur by her admitting to you that she loves someone. It is obligatory for you to repent to Allah for lying, and you must fear Allah, treat your wife kindly, think well of her, and not seek out her faults or look for her missteps.