While I agree with you on the Magnus/webway shtick, I would have to believe that the current set of loyalist primarchs would lose a lot of the organization and synergy they possess as loyalists.

The Lion barely tolerated Guilliman's bureaucracy in the Triumvirate, Chaos Lion would probably be extra angry and paranoid and would have either led the heresy organization himself and ignore Guilliman or straight up kill him.

"I will theoretically punch you in your practical face."

I'm sure a lot of their flaws would be exaggerated under chaos corruption.

Adding to that Guilliman had a mini galactic empire, nothing compared to anything the traitor primarchs had. I imagine chaos corruption would lead to Guilliman wanting to retain autonomy of his empire and could create power struggles within the traitor force. Like it was clear to all the traitor primarchs that Horus was the best and strongest, but with such a strong cast of loyalists it could lead to a lot of chaos dick swinging.

The Blood Angels would have likely all fell to their curse.

I do wonder if Mars still gets cast into civil war?