1) We can be as strong as we damn well like but it wouldn't do squat for us in that situation. Beating the snot out of the Brass Blades, the Crystal Braves and the current lawful leaders of Ul'dah as well as quite possibly corrupt Sultansworn wouldn't have proved our innocence. Infact it would make us look more guilty. This wasn't a fight we could win with strength of arms without dire ramifications. Killing Ul'dah's law enforcers and leaders in mass while we are under accusation of assassination would not only make us look guilty but quite likely turn public sentiment against Raubahn, who is both our friend and ally, a foreigner and someone known to have been close to a high ranked Garlean spy. We would look as guilty as hell.
2) The Echo would be pretty useless. We know we are innocent. We know they are guilty. The Echo isn't going to prove that to anyone except ourselves which we don't need to do. More over, we don't control the Echo. It gives us visions at random. It's not like we can just turn it on at a whim.
3) The Grand Company Leaders had no choice but to leave. There was nothing they could do in that situation which wouldn't make the situation worse. Lets be clear here. We were accused of regicide by our own military unit. They had done nothing at that point unlawful under Ul'dah law to us. Sure they were part of a conspiracy but we had no way to prove that and neither did anyone else in that room on our side. Raubahn killed Teledji unlawfully, without proof of any crime and without any form of trial. It was an assassination. If the Grand Company Leaders had stepped they would have been aiding a person accused of assassinating the sultana and a person guilty of assassinating a syndicate member. The political ramifications would be dire. Even us aiding Raubahn would have done less damage than too foreign leaders assisting directly in a coup.
We ran because until we can get proof that shows we are innocent there is nothing we can do. Slaughter would have just made it worse. Further we cant be sure all the members of the Monetarists were there. They would have a field day undermining our credibility and spreading doubt and suspicion. Worse, if Lolorito got out of that room and I wouldn't rule such a possibility out, it would be an outright civil war. The Eorzean Alliance would be crippled and while Ishgard was ravaged by dragons Garlemald would have a field day taking territory from a weakened Ul'dah. If the Immortal Flames were battling the Brass Blades, the Crystal Braves and a decent portion of the Sultansworn who are in Lolorito's pocket, who would be defending the frontlines?
Actually the fact we retreated is one of the reasons we can still walk the streets of Ul'dah. Most of the commonfolk have doubts about our guilt. If we slaughtered Ul'dah's police force and leaders that would likely change.