Churches are not made from perfect people. Individual Christian traditions are all tainted by human sinfulness. But then again, so are people in general. The quest to find a pure church where your faith is never challenged is a futile one, a distraction from keeping your heart on Jesus through prayer and listening to the Holy Spirit.
The various schisms in Christianity are not nearly as radical as the divisions in theology and practice in other world religions, for instance, in Hinduism or Buddhism. Buddhism in particular is much more diverse in philosophy and practice than is immediately obvious. Most Christians in their doctrines share a great deal in common, the belief that salvation is due to grace given through the Holy Spirit, for instance, or that Christ's death was necessary for human salvation, is universal to most Trinitarian Christian groups.
Sure, but some of those Christian groups, or some of their members, deny that some of the other groups are really Christian and their members can be saved.
I just read last night that the late Coptic Orthodox Pope Shenouda III held Protestants to be on their way to eternal damnation and the high-ranking Bishop Bishoy believes that Catholics are in the same boat. Further, the Catholics and Eastern Orthodox will say that their adherents who convert to the other because they deem it most likely to be the One, Holy, Catholic, and Apostolic Church are in peril of losing their soul too. There are also Protestants who think all of the aforementioned churches are outside the bounds of true Christianity because they deny faith alone, believe in sacraments, and/or follow other "unbiblical" teachings that "deny" the "Gospel".
Where did you read this about Coptic Orthodox Pope Shenouda III? I looked at his wikipedia page and it says that "he was well known for his ecumenism", so the opposite really. Personally, I've always found the various Churches in the east to be surprisingly amicable.
Yes, there might be flashpoints of pithiness here and there, but overall I can see us getting together eventually.
For the ordinary person, who doesn't have the time or inclination to delve into history and theology too much, how can they ever feel anything other than a state of terror over their salvation in the face of the confusion caused by it not being clear which Christian group is right?
Working one's salvation out with fear and trembling = good.
Constant state of terror and confusion = lack of a trust in God's mercy.
I can't imagine that God will take back his grace because you ended up in a Church that believes that Jesus has a two in one nature rather than two natures, or vice verse. I know where you're coming from, but don't over think it. If one thing is clear from reading the Gospels is that Jesus will honor sincere people wherever they are on their journey (Matt 21:31, 25:31-46, Luke 23:43, 23:3).