Your Duty to Assist will not apply; but the reviewer or judge will make sure the VA met its Duty to Assist during your initial claim.
Is there anything I should look out for when I request a call with a senior reviewer?
Yes, you should expect a call at the phone number you gave them or the one your representative gave them on the Higher-Level Review form in which they will schedule a time to discuss your case with you.
What if i disagree with a Board decision?