In reality, they also understood that the Demon League was a great threat. Otherwise, they wouldn’t have gathered at Gu Heng’s place. After Murong Yu discussed the countermeasures, they all feigned indifference so that they could wheedle some benefits out of Gu Heng.
As for the possibility that Gu Heng might double cross them, they had their own countermeasures. If Gu Heng burned the bridge after he crossed the river1 they could band together to cause a disturbance for him. There were so many of them gathered here; if they used their numbers to do something, then Gu Heng’s seat as a successor candidate would probably become unstable.