On the assumption that your pension will come from the Department of Education, the best people to contact is the pensions section there - they are helpful and will reply to you on most of the issues that you have raised - although not on the AVC question, but may advise you on purchasing notional service instead - but it may take them some time to reply to you, as they are always very busy, and I'm sure that things are worse these days with the embargo, plus some staff on term time.
If you are a member of one of the teaching unions then the Union head office should be able to help you - so don't be afraid to ask - you're paying for the services!
Personal opinion - if you can, try to purchase notional service rather than buying AVCs.