This is a cool analysis but I think Ragnar already realized Rollo betrayed them 100% when he saw Rollo in Frankish garb atop the horse.

Fair enough. I think Ragnar is suspicious at that point but unwilling to believe it completely.

Also when Bjorn says he wants to kill Rollo, Ragnar said "good".

That struck me more as Ragnar encouraging Bjorn to hate betrayal in general. Ragnar never did completely trust those around him, except Æthelstan.

He already knew Rollo was against him, the reason he ordered the retreat is because they were getting murked by the Franks all over.

They were getting arrows shot at them, that's true. But even then all the Vikings had to do was travel between the two towers down the middle of the river to lessen the effects they were having on them. The Frankia defenses were believably good but surmountable. If Rollo wanted it to appear like he was helping the Frank's but secretly leaving an opening for the Vikings to get through, that's how he might have done it.

Ships were capsizing, ships on fire, dozens of Vikings killed by bolts, the chain blocked further movement, and a ground force would be destroyed by the bolt rain.

I think the first thing people think about is the time the Vikings fought the two armies by going to one side of the river and fighting them, while those on the other side watched on in horror.

This time wasn't like that, and Ragnar wasn't leading the way.

Ragnar straight up got outplayed by Swollo, he fled when he realized how badly he was getting his ass kicked.

He does retreat and the rest of the Vikings do listen to him and follow suit, that's true any way you look at it.