I'll try and answer as best I can:
1: Galbatorix's first dragon chose him before he went mad. when his dragon was killed, Galbatorix went insane. And then Morzan helped him steal Shruikan, and Galbatorix used dark magic so that Shruikan would obey.
2: He told Arya about this, but he never told Oromis. I don't know why, but he just hasn't....
3: The only thing I can think of here is that Morzan was once an innocent child. Perhaps his dragon chose him, thinking that he was just that.
4: I think that they were in a relationship. The book does not tell us this directly, but it is emplied (by Arya). For instance, there was the flower Faolin made for her, and Arya showed Eragon this just after he tried to take a stab at romance, so as to bring Eragon to heel.
Hope I helped!
