This whole debate hinges on judging characters previous performances. The fact is, we don’t know how much more powerful the Knights are with the Omen boost. There are no rules here, we haven’t seen this amp before. Maybe it makes them ten times stronger and adds abilities to their arsenal. The main problem with the Gorosei suggestion is the implication of Imu falling here, it isn’t going to happen. Never in the history of this manga have the top subordinates fallen while their leader doesn’t, so unless we believe Imu will be beaten here, the Gorosei will not fall either.
What’s been established to be of primary importance is for the M3 to develop the ability to properly counter regeneration, not through gimmicks. It isn't necessary for the Knights to push Zoro and Sanji combat wise, only to serve as training tools. While I agree with the sentiment that the Knights have looked lackluster, I also recognize that Oda has humiliated far stronger and significantly more important characters before.