What’s the long form corresponding to the short form “English Language & Usage”?

Solution 1:

The phrase English language and usage is an example of the syntactic phenomenon called Conjunction Reduction, which omits repeated lexical material in conjoined clauses.
Thus,

• English language and usage

is a reduction, by rule, of

• English language and English usage

This is an ordinary kind of conjunction reduction; English need not be repeated.

Note, however, that this is a syntactic rule, not a semantic or pragmatic one -- that is, it's mindlessly automatic and does not refer to meaning, custom, or context. Let alone disharmony. Syntax, like AI, is all algorithms.

That means that the two NPs English language and English usage have the same syntactic structure and therefore are treated the same by syntactic rules like conjunction reduction, even though the semantic relations between English and the next word are different in each phrase.

It's not necessary to invent imaginary phrases for the reduced phrase to represent; that's just normal behavior for syntax. If you want to give it a Greek name, fine by me.

Solution 2:

The most correct long form of the title of this stack is even longer than your disharmonious suggestion:

English Language & English Language Usage

This stack is for the discussion of the English language as well as discussion of the usage of the English language.