The reason Zoro mentioned enma was because if he runs out of haki and he loses control of enma it will try to force more haki out of Zoro and it could kill him.
Zoro is the one releasing and using large amounts of haki. It's not enma.
Zoro discovered the best way to control enma is for him to release large amounts of his haki.
Enma doesn't release CoC if the user can't control it or lose control of it. It only releases CoA. The manga told us it only releases the user's Ryu or CoA.
Ryu is not CoC or CoO. Ryu is the term Wano citizens use when they are talking about CoA haki.
Zoro's skills with CoC/ACoC has nothing to do with enma. That's why Zoro can use it with all 3 swords at the same time or just 1 sword.
Zoro not knowing he has CoC doesn't mean he couldn't learn to use it later, just like he did. Luffy didn't know CoA haki can be flowed and released from the body. Luffy went from not being able to use flowing haki to using internal destruction haki.
Zoro doesn't have a time limit with CoA/ACoC. The reason he can't use it for long is because he was low on haki at the time. He lost large amounts of haki when he lost control of enma and it released large amounts of his haki. That happened 4 times.
Everytime Zoro lost control of enma it forced out so much of his haki his arm shriveled up. The sword tried to release all of Zoro's haki. The blacksmith told us if he was a normal swordsman his entire body would be husk.
A good example of that is Luffy after he runs out of haki from reaching his G4 time limit.
To put the amount of haki he lost into perspective we just need to look at the amount haki Zoro used from the end of chapter 1033.
Until the beginning of chapter 1035.
As you can see Zoro released large amounts of CoA haki and ACoC haki and caoted all three of his swords with it. Neither his arms nor body shriveled up from releasing those large amounts of haki.
Compare that to Zoro losing control of enma near the end of chapter 1032 where his arm does shrivel up from releasing large amounts of haki.
That means more of Zoro's haki was released in the one scene he lost control of enma then from the time he start using ACoC until he defeated King. He probably defeated King in s few to several minutes from the time he start using ACoC. Remember, he lost control of enma four times. That's four times he lost a large amount of haki.
Zoro has insane amount of haki and there is no time limit.
