So I've been keeping up with the Gurren Lagann discussions at the Animesuki.com forums for a while, and it's really interesting because people are debating whether Rossiu's decisions in the timeskip have been right/wrong, whether Simon has actually been a good leader, the moral dilemmas that they're facing, etc. Here's some of my thoughts concerning that:
First of all, I agree that most of the Gurren Brigade really shouldn't have been in the government. I'd say Rossiu and Dayakka are the only ones suited for the job, because they actually have prior experience. For good or ill, Rossiu had spent a great deal of his childhood helping out the leader of his own village.
Simon, on the other hand, spent his childhood digging holes and piloting a mecha. He was trying his best, but he just didn't have the necessary experience to lead a government in times of peace. Even when he was leading the Gurren Brigade as a kid, it was a passive sort of leadership- Leeron suggesting he give out an order in episode 12, for example. Simon was the best and most reliable fighter: he had the power, instincts, and determination to pull through in battle and people looked up to him because of this. In all other matters, he was capable of good decision-making, but not to an exceptional degree.
My point is, it's kind of difficult to argue whether Simon would have been a better leader than Rossiu in the current situation, because he wasn't given a chance to prove himself. He went out to do what he's best suited for (fighting the enemy) which didn't turn out too well, only proving that one should not fight in a crowded city the same as you would in a battlefield. And then he got majorly distracted by personal matters (Nia), which also didn't turn out well because he was absent at a very crucial time and his conversation with Nia got broadcasted everywhere. And then before he could do much more, he's arrested and put to death.
Now it's obvious that eventually (in a few episodes probably) he'll become a great leader, from what we see in the opening scene of the first episode. He's got the capability for it. Of course, that opening scene was also during a battle...
Simon as he is now makes a lousy commander-in-chief. He's more eager to go out and fight himself instead of ordering other people to fight for him. Simon makes an even more lousy presidential figure, because while he cares about the people and keeping them happy, he's not really aware of how to go about doing it and how fickle the minds of the public are. What Simon is good at is being a general, and such a position isn't quite needed in peacetime.
Rossiu is more suited for leading the government. I suspect the reason he's become such an "asshole" right now is because he's been having to deal with the incompetence of people like the twins for seven years. He can do the job better, so he's given all the important responsibilities, and because he can do the job better, this inflates his ego and makes him even more paranoid that others aren't doing their jobs. He starts moving on his own because he feels it isn't worth discussing things with his peers anymore. By now he thinks that he's the only one who can do things properly, and once he found a chance, he takes complete control to make sure that things do turn out alright.
He turned out to be wrong. Rossiu can't do everything on his own. He needs Simon now like Simon needed Rossiu during those seven years. And none of this makes up for his complete lack of remorse in sentencing Simon to death; he could have at least apologized, but instead he just went on about how Simon needed to be sacrificed. Gee, that's sure nice. Friends for seven years and not even an "I'm sorry for having to do this to you."
Rossiu could have been a good leader if Simon had chosen his officials more carefully, instead of just handing out positions to all his comrades. If that had happened, by now they could have been working on evacuating Earth and fighting the Anti-Spirals together, instead of one shouldering the burden and the other unfairly condemned. And Simon could have been a better leader if Rossiu didn't keep important information from him, and again they could have worked together to take care of the problem before it got worse.
So really, they're both at fault. For Simon, it was an innocent mistake due to lack of experience. For Rossiu, it was forgetting his old loyalties and overestimating his own abilities.
...Kinda off-topic, but I keep wondering what Nia would be doing if she hadn't turned Anti-Spiral. Simon gets arrested? Rossiu gets bitchslapped. Simon gets an unfair trial? Nia goes OBJECTION! and gives a long speech in court. People start rioting in the streets? Nia makes a public announcement that they're sorry for unknowingly dooming the planet and that the government will take care of things, so please calm down everyone~! Man...things would be much better with Nia around. :(
And LOL, how would she react to seeing her father's head in a jar?