In the end, Shane's explanation was accepted by everyone.
It wasn't that everyone trusted Shane enough to believe him, it was the support from Vivian and others that made it difficult for the crowd to reject his explanation.
Even if they did not trust Shane, people like Beren still had quite a bit of faith in the members of Vivian's Team.
So, just like with Riley, since everyone from Vivian's Team vouched for Shane, accepting his statement became a matter of course.
Besides, what else could they do if they didn't believe it?
Regardless, it would have been difficult for Beren and the others to contend with Shane, who had even defeated Beddo.
Even if Shane himself claimed that his strength was only revealed after meeting certain prerequisites and conditions, Beren and his group weren't sure that those restrictions and conditions wouldn't be met if they decided to challenge him.