Mihawk had to join an alliance to get away from them. The fact that he had to do this, shows that their strategy was effective, no matter how much you pretend that not to be the case.
The Emperors do not scare them. Up until their fall, the WG intentionally kept the Emperors around for the sake of the balance of power. Nothing more, nothing less. If they really wanted the Emperors gone, they would have been. The Admirals are not even supposed to be dispatched until given direct orders from Akainu or the others that rank above him lol.
Not sure what you mean by "they can't win or do anything relevant" either. Because the Marines aren't the ones losing out. They have the Warlords on the ropes and scrambling. Besides Koby getting kidnapped, they successfully caused BB to flee with just the seraphim, who are WEAKER than the admirals lol.
No lol.
Isn't canon. So the events that happen during it is irrelevant to the discussion here.
None of the admirals were scared of Whitebeard. Each admiral pulled up on WB without issues, the only reason the old man lasted as long as he did, was because his crew jumped in multiple times to keep the admirals at bay for him lol. When WB pulled up for the 1 v 1 vs Akainu, the admiral had the upperhand and eventually put a hole in his chest lol.
Sengoku stopped the war because they already killed ace. If they really wanted to take down Shanks, he would have been taken down. There were 3 admirals, Garp and Sengoku. They would have had loses, but that would be an assured victory for the marines. The WB pirates were never winning that war lol