The Sky Path example doesn't work. If Doffy wasn't using Haki, his strings would harmlessly pass through Smoker. Obviously, Doffy isn't trying to slash the clouds he travels on, so his Sky Path and his offensive techniques work differently.Haki was covered around Luffy's body.
Haki has been yet to be shown extend off a devil fruit user.
Example: When Akoji shots Ice sabers, it doesn't get covered in Haki.
It's just an ordinary Ice sculpture.
•Vergo can't extend his Haki on to his bamboo stick without hold it.
•Pica can't cover assimilated stones that he's not touching in Haki
•Akoji's Pheasant is independent from the caster cannot be covered in Haki.
Besides those, Haki has yet to be shown covering smoker's smoke and Sabo's fire.
The only person so far is luffy, which is understandable because his devil fruit doesn't take a different form.
All it does it extend/stretch his already existing body.
But one thing I did notice is that luffy has never expanded/stretch his body after applying Haki.
He always stretch/expand first before applying Koka Haki, and there could be a possibility that this spiritual armor restricts it.
Doflamingo uses skypath (strings to attach to clouds) in the air, and we know for sure that he is not using Haki everytime he glides from one cloud to the next.
Since Smoker carries the same properties of a cloud, his strings wil get smoker without Haki seeing how sky path works.
Smoker was just an bad match up against smoker, and clouds are Doffy's strength.
The part about Vergo's bamboo stick not having Haki on it when it's not in contact with his hand is also a bad point. The Kuja Warriors shoot arrows laced with Haki. When the arrows are soaring through the air and hit their target, they still have the Haki laced on them despite not being in contact with the Kuja's body still, so that disproves that point.
Aokiji's Pheasant Beak didn't have Haki on it because Oda hadn't introduced Koka yet. Aokiji was using the invisible variant of Haki, the same way:
1. Marco did when he kicked Kizaru
2. The same way Whitebeard did when he injured Akainu
3. The same way Rayleigh did when he showed Haki to Luffy and dueled Kizaru
4. The same way Hancock did when she kicked Smoker
5. The same way Vista and Marco did when they slashed Akainu
Another example is Jozu using Haki and his diamond powers at the same time to injure Aokiji and Crocodile.