CW Anakin beat CW Ventress because he was pissed off and used the dark side.
Rots Anakin is much, much stronger, whereas CW Ventress is still relatively weak. She doesn't reach her height until Obsession.
Obi-Wan vs. GG might be slightly different because GG wasn't at his best in Rots. But even then it doesn't matter.
Rots Obi-Wan uses his mastery of Soresu to hold off GG long enough for Anakin to make quick work of Ventress, then they double team GG and finish him.
He... has no defense? Just because Anakin's form of lightsaber combat does not specialize in defense like Obi-Wans does, that doesn't mean he can't defend himself. Anakin is certainly as skilled with a lightsaber then Obi-Wan is, moreso actually. And remember that even Dooku was superior to GG in lightsaber combat, so Anakin definitely has a chance. Let's also not forget that as of Rots, both Anakin and Obi-Wan are no push-overs with the force. They are both superior to GG and Ventress in force abilities.
So if Obi-Wan fought Ventress and Anakin fought GG, Anakin could mand-handle GG with the force while Obi-Wan either defeats or holds off Ventress. If he kills Ventress quickly then he double teams GG and kills him, or he holds Ventress off while Anakin beats GG and comes over to kill Ventress.
Both Anakin and Obi-Wan are individually better then the other two, so the pairings don't matter.
Yeah Asajj and Grievous hate each other, Asajj is probably pissed off that Grievous is a higher rank than she is and Grievous, idk if he hates her but in The Dreadnaughts of Rendili he gives her an order and (can't remember exactly) but Asajj wouldn't listen to him and she was like "Try to stop me if you want to, but I won't obey you." or something slightly similar to that.