I think Guld was selected because, border-line insane or not, he was the only one who could fly the prototype.
I have not seen the OVA for awhile, but I rented the movie a couple months back. The topic starter is right - the "plot" of the movie is impossible to follow. It was weak in some places in the OVA, but the character's motivations in the movie are impossible to understand. Example: Even with Guld chancing him, how can Assamu possibily justify trashing major buildings in Macross city? What is Sharon Apple trying to do at the films climax? These are not adequately explained in the movie.