The type of handicaps Sanji faces come NOWHERE near Zoro's. The only handicaps Sanji faces is not being able to hit women, but these aren't things that causes to him to struggle to walk/stay conscious for weeks.
Luffy's > than both yes, but I don't think it's overwhelming. I imagine it's like the relationship the Akainu/Aokiji/Kizaru/Sengoku/Garp when it comes to strength. Anybody can beat anybody depending on the circumstance.
You miss when he cut Monet's cheek? Or did you mean Observation Haki? He showed glimpses of it against Daz Bones. He conclusively used it when he, Luffy and Sanji sensed Caribou trying to kidnap Shirahoshi.
There's no question in my mind that Zoro will get COC. Maybe Sanji will as well, but I feel more comfortable about Zoro's chances. The main purpose of COC is imposing willpower. Those with the greatest ambitions, willpower that other characters acknowledge and imposing their will/spirit unto others are ones with this type of haki. Since the beginning of the series Zoro, like Luffy, has been noted to have extraordinary willpower, even Sanji was shocked at it when he first met them. Zoro has also been able to impose his will, not like Luffy with making people faint, but numerous people have wound up being uneasy/frightened by him. Even Monet, a person who serves under Doflamingo, was so scared of Zoro that she couldn't even use her Logia powers right.
I also believe Oda is making a bit of a parallel with Zoro and Rayleigh. Both the first member of the crew, both swordsmen, both have a scar on their eye, both have an "evil" motif (Dark King Rayleigh/numerous demonic references from Zoro). Zoro getting COC like Rayleigh.
Zoro getting COC wouldn't fukk anything up. COC isn't required to be a top tier. As it remains right now, none of the current/ex Admirals (except Garp) nor Blackbeard have COC. It remains to be seen if Big Mom/Kaidou/Mihawk have it as well. It's safe to say that regardless of what haki they do have Zoro/Sanji will match or surpass at least the Admirals/Mihawk and the characters with COC like Hancock/Rayleigh/Chinjao/Ace. If all those other characters DO wind up having COC, Zoro/Sanji will get it as well.