Pretty sure it is 35 years needed. I was born in 1955 and just 6 weeks too young to be allowed to buy make up years which is what older folks were allowed to do - they weren’t simply gifted extra years. At least that is how I recall it happening.
Not sure about your other questions but on your last point, the FBU agent told my wife to complete SSA 308 but that they would coordinate with DWP to confirm the numbers.
[For the benefit of the OP, but relevant to your last point]
My recent experience is that the communication between FBU and DWP is not exactly "efficient". FBU have had to resubmit my PODSU-231* form once already, because of no response from DWP. *PODSU-231 is a form used to get confirmation from DWP that the years of NI you say were voluntary/non-employment based were voluntary/non-employment based.
In addition to that, FBU have simply not been answering the phone for several weeks now. When I contacted them by message to ask if DWP had responded yet, I received the following reply: "The Windfall Elimination Provision is not currently applied to your benefits [I already knew that, as evidenced by my original message to them]. We have received the PODSU231 from the DWP and have provided it to the processing office for their office to determine the WEP offset going forward.Once they have completed the above actions they will notify you in writing."
- in other words, keep waiting. I received the above message from FBU 3 months ago. So far, it's now 14 months since my SSA pension should have been WEP'ed and when they eventually get around to processing things, I calculate that I'll probably receive nothing from SSA for many months, until it all "catches up."