NEM is not the new pensions scheme, FAFPS is. You would probably get better clarity if you divorced the 2 and treated FAfPS as an individual entity. NEM is a look at ur complete offer in terms of service (housing, pay, allowances etc)
I think you concern centres around how protected is your afps 75 pension post 2015? Have a good read of the blurb that is out there as it is reasonably well explained. In essence, you retain your rights to any gratuity and pension on the 75 scheme until you retire and collect the gratuity on departure and your pension at 55. However from 2015 you cease accruing 75 pension bits and start accruing new pension bits on the new scheme (afps 15?) and will collect this at age 60 ( I think). In essence you will get 2 pensions, one at 55 and one at 60. If they amalgamate into one payment I am unsure. Bottom line you will still get your gratuity and immediate pension on the 75 scheme, just that it will be a tad less post 2015 as you start the new scheme then. Hope that makes sense. If you were 45 on 1 apr 12 then ignore the above as you will stay on the 75 scheme until you go.