When traveling, can I delay ‘Isha until after midnight? If so, can you also delay Maghreb until after midnight, to combine it with Isha?

No. A traveler combines the two, so in delayed combining he prays Maghrib within the valid time of ʿIshāʾ and then ʿIshāʾ, and he does not push the combined prayers past the chosen time of ʿIshāʾ.