[Spoilers Extended] Would it have changed anything if Barristan sided with someone else during the Wot5K?

As we all know, Barristan Selmy, Lord Commander of the Kingsguard, was dismissed by Joffrey after the death of King Robert. Afterwards there’s lots of speculation in King’s Landing about where he’d go. It’s assumed at first that he would join the cause of Stannis, Renly, or Robb, but instead he travels to Pentos and then to Qarth to meet Daenerys Targaryen.

What if he had decided that Stannis were the rightful heir and chose to support his claim? Or what if he sympathized with the Starks after what happened to Lord Eddard? Would his presence change anything?

I’m not sure if it would or not. Obviously Barristan Selmy is a figure who commands a lot of respect and would add legitimacy to, say, Stannis or Renly’s claim against Joff. But other than that, how can he really change things on a bigger scale?

If he supported Stannis, his presence on the battlefield probably wouldn’t be enough to change the outcome of the Blackwater, unless Selmy’s allegiance were to strengthen the loyalty of the Storm lords. Even then, the Reach had already been turned in Joffrey’s favor, and they caught Stannis’s army from the rear well into the battle.

If he supported Renly, I don’t see what he can really change. Renly is still assassinated by Stannis’s shadow. The only change I can see here is that maybe after Renly’s death he leaves with Catelyn and joins Robb? But even that seems unlikely since he’s a stubborn man of honor. I doubt he’d believe the story of the shadow magic. He would probably think that Brienne killed Renly.

And finally, if he joined Robb for some reason, I’m similarly unsure of what he can change. Maybe he’s involved in the Whispering Wood as part of Robb’s personal guard (if Robb even trusts Joff’s former kingsguard that quickly) and is able to prevent Jaime Lannister from slaying Lord Karstark’s son(s). After that he probably heads west with Robb, and doesn’t really change anything. He probably dies or is taken hostage at the Red Wedding.

Am I missing anything? Is there any important information that Barristan knows that could’ve changed some crucial event at some point? Would be interested to hear others’ thoughts on the possibilities here.