A question about "but not" as coordinating conjunction

It is grammatical. Also, you seem to be confusing the issue with the coordinating conjunction. The coordinating conjunction is superfluous and can be eliminated entirely.

Example:

• I made him do it, not her.

Your adding "but" isn't necessary, and this has nothing to do with coordinating conjunctions. It is completely grammatical to add a contrasting "not" phrase, or complement, to the end of a sentence. It's done with exceeding regularity.

Click the following link, and scroll down to rule seven: http://grammar.ccc.commnet.edu/grammar/commas.htm

As an aside, you'll notice that I didn't use the following example of yours in my above answer:

a. You can turn everybody against you, but never your boss.

This is a poor example because it's unclear what it even means. It can mean that you can never turn your boss against you. It can also mean that you can never turn everybody against your boss. Owing to this lack of clarity, I chose not to use the example in my answer. Know, however, that the "but" you use in it is just as unnecessary as the "but" in the example of yours I did use.