Hmmm well it really just depends on how exactly AdCoC counters the Holy Knights.
If AdCoC can straight up kill/beat them, then (fruited) Loki dominates Harald with no issues. If AdCoC can’t really beat them but can only impair their regen, then I guess Harald could eventually win, but it’s hard to see how if his body is destroyed and he can’t regen or has issues healing.
The overall power of the Holy Knights and Gorosei depends on the full extent of their regen.
Regen is the main factor but not the only one.
The Holy Knights and the elders have devil fruits,weapons and haki.
Even a guy like Sommers could protect himself to some extent against Gaban thanks to his haki and his fruit.
That's why Harald, Xebec and the elders are so strong. On top of the regen they can use conqueror coating,have legendary weapons and/or broken devil fruits.
First of all Harald threw his sword who was then stuck in Jarul's head, he didn't use CoC against Loki (seemed to have discovered that Loki was a conqueror few minutes before his death). On the other hand Loki just gained a legendary weapon and a legendary devil fruit.
People ignore the plot. Gaban, Shanks,Roger and Garp all managed to hurt Xebec and Harald.
None of them was allowed to strike against an opponent who conciously didn't defend against a devastating strike tough.