ことはない => Not necessary N3
Needs, Requirement
それはそうと => By the way N2
Miscellaneous
ことなく => Without doing A once (expressed strongly) N2
Will, Decision, “Doing”
ことなしには => Without doing A, B N1
Conditional, While
はともかく => Whether or not A, B (is true) N2
Based, Focused, Depending on
に[越「こ」]したことはない => It's better if A, it's better if it's A N1
Comparison/ Exchange
に[限「かぎ」]った[事「こと」]ではない => It's not limited to A, it's not only A N1
Degree/ Level/Amount
ことは~が => Although A is true, C N3
Contrast
とは[限「かぎ」]らない => Its not really A N2
Information, Report
ないことはない => There is a (small) possibility of A. A is not impossible. N3
Potential, Possibility
とはくらべ[物「もの」]にならない => It's no match for A N1
Comparison/ Exchange
てこそ => Now since A... N2
Reason, Result, Response