Is it valid to pray Maghrib during a seminar while sitting and facing away from the Qibla, for fear of its time expiring, knowing that it was made up later?

Your prayer while sitting and facing away from the Qibla is not valid. You were correct in making it up, because standing and facing the Qibla are two pillars and conditions for the validity of prayer that are not waived for one who is able to fulfill them. Being preoccupied with the symposium is not an excuse. You should have performed the Maghrib prayer at its prescribed time, because delaying a prayer beyond its time without a valid Shar'i excuse is impermissible. Whoever does so must repent to Allah.