The manga disagree with your statement, "we can't tell if Zoro can use CoC at will". He can and there's manga proof backing out up.
Zoro not using CoC when he's just blocking attacks.
Zoro releases CoC haki as he prepared to attack.
Zoro use CoC haki with just one sword without needing to release overwhelming CoA haki.
Dragon style is Zoro releasing maximum or overwhelming CoC haki.
As you can see, Zoro has full control over his CoC. He can use CoC with any sword and without needing to release overwhelming amounts of CoA haki. He can even increase his CoC haki output.