でなくてなんだろう・にほかならない: the difference
Meaning
If … is not … then what is?
Past papers
Example
あの子は新人だというだけでいじめられている。あれはパワハラでなくてなんだろう。
She's being bullied just because she's new. If that isn't power harassment, what is?
にほかならないN2
Meaning
Nothing but, None other than, Simply
Past papers
Tested 2 times in past papers
Example
第一にすべきは人命にほかなりません。
The first priority is nothing other than human life.
How to tell them apart
「でなくてなんだろう」: a rhetorical question, emotional; for heartfelt emphasis, praise or angry blame.
「にほかならない」: a calm statement that 'it is nothing other than', used in essays to name the real cause.