First of all, as you probably know question words like 誰、どこ、どれ、何、なぜ etc cannot be part of a topic (cannot come before は).
あの方 is your "topic" because you are moving the conversation topic to them when you are asking who they are.
While 誰があの人ですか is not technically grammatically incorrect, it is not properly worded because あの方 is your topic, and saying 誰があの方ですか sounds as if you're trying to say 誰はあの方ですか and remembered to change it back to が at the last minute.
Not technically grammatically incorrect, but here you would usually say 誰か、車がありますか？/誰かに車がありますか, because you're looking for a person who has a car, not the person who has the car.
Now, say you're looking down a car that is double parked and are angry. 誰の車ですか！Now this would be a little strange, but say you are in an isolated situation where there is a car and you are trying to figure out whose car it is and it can only be one of the four people sitting in front of you.
だれに車がありますか？"Who is it that has a car?"
だれかに車がありますか？"Who has a car?"
That's the nuance/difference there.
As for an example that uses 誰が... (unless you're using it in place of の - 誰がために働いていますか - "For whom are you working for?") - it would be hard to think of, as the topic is always the person you are asking about. If you insist on having 誰 come first: 誰ですかあの方は？ but this really does make you sound angry because of the stress it puts on the 誰 ^o^;