For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
820-3536 and 820-3476 are interchangeable (as far as I know--we use the same test cases for them without issues--someone will correct me if I'm wrong, but we've noticed there are no issues). The 820-4924 uses a different trackpad connector, so it's not interchangeable.