(...) The dragon was made out of pride but I don't see how that pride lead him to do an unneeded sacrifice when all he could just rain down fire and kill everyone with his dragon to make his point. Cory is a guy, if he is show boating, wants to be seen. If he did come there just to kill the divine and sabotage the conclave, he would make it know to everyone there he's doing it and to kill the divine right there infront of a crowd to show how superior he is.
But that would be counterproductive

A murder (or an explosion) with unknown culprit gives us more chaos. A blighted dragon ridden by an ancient darkspawn claiming to be a magister that assaulted Golden City is a good way to make all those idiots forget their differences and unite.
2. Attracting spirit is not new but it's rare. It does not happen unless one is a mage, one is using magic
Oh, that hypocrite Cassandra, I knew she was secretly a mage!
or the veil is thin
You mean, thin like in a temple sitting on top of mountain of Lyrium, housing for a couple hundred years an urn of mircale-working ashes protected by spirits and powerful magic?
and even then what commonly comes are malevolent demons not benevolent spirits. And Yes a spirit coming would mean Justinia is special in some way, especially is the spirit is one as rare as faith. Even more so in the place of the fade controlled by an all powerful demon.
First of all, we don't know what spirit it was (we don't even know IF it was a spirit, really), second: if it was a Faith, where do you expect to see a Faith spirit if not in the middle of an event that half the Thedas looks too with hope? Seriously, find in history one situation where people were praying for success of one singular localized event to similar extent. For all we know Summit could in fact CREATE a Faith spirit rather than attract some of them.
1. But the conclave did not blow up because of the conclave itself. It blew up because some picked up something filled with power that they did not understand.
That's your assumption, not an argument. You assume that the Conclave was irrelevant and disrupting it was just an accident. But when you try and base your arguments on your assumptions, that's just circular logic.
2.That's a stretch with all that fear going on there. Added with the fact that the part of the fade they fell to in the explosion was Nightmares. Were dealing with fear, rage, despair and desperation at that conclave more then faith even before the explosion. Sorry but a spirit of faith being there is extremely strange.
Now you're just being ridiculous. Just as fear does not preclude faith, so does an event people's emotions concentrate on draw many kinds of Spirits. And Justina herself (along with those that trusted her) was hopeful. The whole event was founded on basis of hope and faith that were supposed to overcome fear and anger. That's what the whole Summit was all about.