It's lww to me Gaban and Shanks would not try to wash the name of an innocent, for Elbaf Loki killed Harald and 113 soldiers out of sheer hunger for power and has been chained for the last 6 years in consequence. Especially with how Oda been portraying Shanks.
It's possible Loki told them to not say anything without his permission, but then I wonder what their relationship is really like.