Sybe:
Perhaps this will help you in reviewing your question:
Nibbana as Nibbana can have no beginning, and, of course, no end as well. An infinite number of beings have attained it (in the past), and an infinite number will attain it in the future. It was always there to be attained, though for any particular being ‘It does not exist’ until attained. So, It has a beginning only as far as any being in particular is concerned.
I leave to Lal the fuller explanation as well as your last question.
Metta