I'm late, sorry.
Yes.
No, that's a licensed profession in most countries.
Yes.
Nope.
I wouldn't call myself a PROFESSIONAL football player, but anyone who plays football is a football player. It's LANGUAGE.
And of COURSE this is gate-keeping. There's a term, and you want to define it such that some are in and some are out. Your "football player" stuff only emphasizes that.
The only question I have, going back to the original post, is why you feel there has to be a line drawn at all? Who benefits from such a thing?
If I recommend drawing that line so that you, personally, are considered "not a game designer", would you be okay with that? Because I assume you want the line drawn so that the "wannabes" and "yahoos" are on the other side from yourself.