Does a conditional divorce pronounced as an oath, contingent upon entering Messenger, take effect if the wife enters it forgetfully, knowing that the husband intended to threaten and not to divorce?

You are not required to divorce if you were aware of what you were saying. If the reason for the oath ceases to exist (the wife speaking with a non-mahram man), then no divorce occurs, because the inciting context that led to the oath has an effect here in preventing the divorce from taking place. Some scholars, such as the Shafi'is, state that no divorce occurs if your wife speaks with you, having forgotten the conditional divorce oath.