Is a person considered sinful for delaying the prayer, despite their eagerness to perform it on time, if they were unable to pray in a designated place and did not know the direction of the Qibla, which compelled them to combine the Maghrib and Isha prayers upon returning home?

You erred by missing the Maghrib prayer. Ignorance of the Qibla is not an excuse; rather, it was obligatory to ask, or to exert effort in determining its direction, or to pray in any direction when unable to exert such effort. You must repent to Allah Almighty with remorse and resolve not to repeat the sin.