Is a man's 'Isha prayer valid if he joins the Imam for 'Isha prayer with the intention of combining it [with Maghrib] after having missed Maghrib prayer with the Imam and prayed Maghrib alone?

For the validity of combining prayers, it is a condition that the second prayer be performed in congregation during the time of the first prayer, even if the first prayer was performed individually. This is the view of Sheikh Sulaiman al-Jamal in his commentary on Fath al-Wahhab, and also the Maliki school, who permitted combining for someone who prayed Maghrib alone and then found a congregation praying 'Isha, so he joined them.