What is the ruling regarding someone who caught the Isha’ prayer while not having prayed Maghrib yet, then prayed Maghrib and Isha’ combined and shortened after performing the rituals of Umrah? And what are the consequences if what he did was incorrect?

Scholars have differed on this issue. The preponderant view is that if someone remembers a missed prayer while performing a current prayer, they should complete their current prayer, then make up the missed prayer, and then repeat the current prayer as a recommended act to maintain the order. Therefore, what you did by praying Maghrib and Isha afterwards is correct, and nothing further is required of you.