Is it permissible to defend a client whose innocence is highly probable, despite some doubt concerning the accuracy of certain statements he made, and in light of him being subjected to an attack by specific adversaries?

The fundamental principle regarding advocacy is its permissibility as long as it aims to restore rights and lift injustice through truthfulness and fairness. If you are certain or highly confident about the truthfulness of your client and that he is in the right, there is no harm in defending him, even if his opponent is from the Rawafid (a derogatory term for some Shi'a groups), while being careful not to let your dislike for the opponent lead you to injustice, because justice is obligatory even with a disbeliever and an innovator. Your strong presumption of his truthfulness is sufficient for you to represent him, until proven otherwise.