Would the GOP establishment abandon Trump in this scenario?

[Ferguson97]

They'd either frame it as self-defense ("violent thugs trying to attack the former president") or they'd condemn the murder but pin it all on the murderer themselves, and insist that Trump did not really mean what he said.

You're forgetting that conservatives aren't operating in the same reality as the rest of us. If the situation isn't favorable to their agenda, they'll try and reinvent the narrative of the situation.