The opposite of labile is stabile.

Probably a more common context for these words to occur is medicine, or more broadly toxicology, where you might have heard:

Heat labile vs heat stabile

The sentence is grammatically correct because as per the rules of English grammar the nouns/verbs/other parts of the speech are in the correct locations.

The overall sentence is syntactically incorrect because you forgot to put a question mark at the end.

The sentence is semantically correct (which you actually meant to ask), though with a better alternate, as explained by the sibling comment.

Maybe but it's awkward as heck because of the double negative. "Nothing less labile" sounds a lot better.