What should one do who missed the Dhuhr, Asr, and Maghrib prayers, and then attended the Isha prayer in congregation: should he pray Isha with the intention of Dhuhr, or should he pray Isha and then make up all the missed prayers, including Isha?

It is permissible for someone who missed Dhuhr prayer to pray it behind someone praying Isha. It is preferable for him to pray with the congregation the prayer they are establishing (Isha), and then pray the missed prayers (Dhuhr, Asr, and Maghrib). He is not required to repeat the Isha prayer afterwards, because the order between prayers is waived due to an excuse, and witnessing the congregation is considered one of these excuses.