Please find the answers to your questions:
1) Will someone at the office sign on receipt of a registered letter, so the Royal Mail will confirm delivery?
All post is received at a centralised point for scanning onto our system. If a signature is required for the post they will sign it when the royal mail is received.
2) Due to the long delays, does the deadline of 31 July 2023 for making NI back-payments from 2006-2016 relate to the date when the document is scanned and in the system, rather than any decision being made? i.e. as long as my CF83 is in the system before 31 July 2023 it's not too late:
You will not be penalised for the delays in us replying to your application as long as its received before the deadline of 31/07/2023. If you are accepted to pay voluntary NICs you will also be given 8 weeks to pay the rates quoted in our reply.
3) I was advised to apply for class 2, on the understanding I may have to pay class 3. Once my CF83 is being dealt with, will I receive clear notification of what to pay, or do I need to re-enter the system again?
NI38 information on the GOV.UK
website provides the circumstances that you must meet to be accepted for both Class 2 and Class 3 rates of the voluntary NICs. You will receive a shortfall letter providing what you can pay and this will also give the information on how to pay.