In my opinion, conjugating the object of a sentence instead of the subject is not an idiom--it's a grammatical error, related to the most basic structure of sentences (subjects and verbs).

Yes, it sometimes sounds awkward when used correctly, but that does not make it any less correct. If the writers at the GMAC want to consider that an "idiom," then that's fine, and of course GMAT students should take note. But that doesn't mean that I have to personally agree with them.

Although if I were forced to choose, I still would select Choice B (prioritizing correct grammar over awkwardness), I would say that there is no correct answer to this question, because Choice B is awkward (albeit correct) and non-parallel, and Choice C is grammatically incorrect. GMAC says (emphasis mine) "Select the answer that produces the most effective sentence; your answer should make the sentence clear, exact, and free of grammatical error. It should also minimize awkwardness, ambiguity, and redundancy." I don't see an answer choice here that fits all of those descriptions.

Would you say "a group of exchange students are coming to visit."? No, you would say "a group of exchange students is coming to visit, because "a group" is the subject, not "students." What's the difference between "a group" and "a proportion?" Essentially, nothing.

I think we need to stop treating the GMAC question-writers as infallible gods, and realize that they are also human, and whether they will admit it or not, they make mistakes sometimes too.

From the 2016 Official Guide, page 667:

"Sentence correction questions may include English-language idioms, which are standard constructions not derived from the most basic rules of grammar and vocabulary, but idioms are not intended to measure any specialized knowledge of colloquialisms or regionalisms."

In real life, when having conversations, I do prioritize avoiding awkwardness over grammatical correctness (for example, "It's me"--incorrect but accepted--vs. "It's I"--correct but awkward--when speaking on the phone), and I think that most of us do to avoid seeming pretentious in the company of our friends who are not necessarily grammar experts. But on the GMAT, the language provided by the GMAC suggests that correct grammar should be prioritized over a lack of awkwardness.