必ずしも~とは限らない => Not necessarily; not always N3
が必要 => need; necessary N4
必要がある => Need/Need to N4
~やむをえず  => unavoidable, no other way to do something N2
Necessity, obligation
や => ... or... N5
Line up, list
や否や => as soon as; the moment ~ N1
や、やいなや => As soon as A, B N1
Time, Situation
や => and; or N5
…ず、…ず => No... but also no... N1
Explain
やっと => Eventually... N4
…とやら => heard that~ N2
Heard
またもや => again N3
Repetition, habit