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.
I'd also like to learn more about the Higher-Level Review. Is there a spot on your website where you keep a list of frequently asked questions?
Yes there is, Go to FAQs and there you can get answers to some of your questions
Can you please tell me on what should I expect if I request a call with a senior reviewer regarding my case?