How difficult opening a 100% blockage is all depends on the cardiology team. I have been through this where 9 cardiologists turned me down whereas the 10th said he could do it with his eyes closed. Saying this, he still had a problem where the wire slipped and went through my artery wall which had to be quickly stented to stop the bleeding. As stated, your husband must have good collateral supply for him not to have a heart attack, but I would go further and say for him to be alive. If stents are offered as opposed to bypass, I would want a cardiologist with at least 20 years experience in such blockages. What you don't want is a cardiologist removing half of it and giving up.
The blocked arteries have the higher priority over the afib.
How do you know that the arteries are fully blocked and how do you know that they are for 10 years?
Either the vessels are not completely blocked or some collateral vessels must have opened to supply your heart with the necessary oxygen, otherwise your husband would have suffered a heart attack.
It is hard to say how difficult it is to open the vessels. I may be easy, but can also turn out to be impossible. You will have to wait and see what the catheterisation brings. Good luck!