Is the healer's ignorance of what the shaykh will say—"Bakh Bakhun" (Excellent! Excellent!) and "Na'am" (Yes!)—after reciting the Quran and Surah Al-Ikhlas, considered a justification for the absence of sin?

The exorcist's utterance of "bak bakhan" after the Ruqyah, if he considers it part of it, is incomprehensible speech, and one must be wary of Ruqyah performed with incomprehensible words. As for his saying "yes" after each verse, it is not from the Sunnah and could be considered an additional innovation if it is consistently adhered to. It is not permissible to resort to charlatans and sorcerers, but there is no sin upon you if you were unaware of the aforementioned person's true nature.