I have a feeling that he was intentionally displayed as childish near the end of the series, petulantly asking why the Elves favored Nasuada over him, as a rushed way to make the readers conclude that he was an unfit ruler. Despite his demeanor, he & Surda really were the group most appropriately poised to take command of the empire imo. The Varden was effectively a rag-tag army of revolutionaries, terrorists even, that happened to be at the forefront of most battles; while on the other hand, Surda was an established nation that not only supplied most of the Varden, but risked the most for the downfall of Galbatorix & then received a raw deal afterward. In fact, you might consider them a last remnant of the original Broddring Kingdom that resisted the Empire, which should’ve rightfully been restored to its rulership, & certainly would’ve enjoyed more support from the Broddring Empire’s citizenry than the Varden (consider that it was not Surda who had harassed the Empire’s subjects for so long or initiated the upheaval).
So with this is mind, I wonder how things would’ve turned out if Orrin had become king in Ilirea? Or if he’d married Nasuada? Even with his personality, he was an able leader & certainly intelligent, so I don’t think the outcome would’ve been so bleak. I definitely don’t think he would’ve let Teirm stay independent, especially when they were completely surrounded & without any allies. What do you think? Do you think Orrin would’ve been better than Nasuada?