Is a divorce pronounced by a man, who uttered it due to a jinn possession by an envious marid-type jinn, valid? And is it permissible for him to take his wife back after they both learned of their affliction with envious jinn possession?

The divorce you uttered to your wife, while being fully aware of what you were saying, and acting voluntarily without coercion, is effective, even if it was due to demonic possession (mass) or sorcery (sihr). However, if you uttered it while unconscious or under duress, the divorce does not take effect. We advise you to resort to the Sharia (definition) court to rule on the matter. We also warn against the impermissibility of going to sorcerers, and that the cure for demonic possession and envy (hasad (definition)) is through legitimate ruqyah (definition), reliance on Allah, and consistent recitation of dhikr.