I have a 100% occluded carotid artery. When should I consider surgery?

Never, most of time. Total occlusion is rarely treated and if treated it is with temporal artery to middle cerebral artery bypass (done very very rarely). Just follow stenosis of other side.
Don't. If the artery is 100% occluded surgery or a stent is not neccessary or possible. The artery when 100% blocked occludes up to the brain and cannot be opened up. The risk of carotid blockage is when it is severely narrowed ie >80%; then a piece of plaque can break off, lodge in the brain and caues a stroke. If the artery is 100% blocked nothing can break off. The other arteries should be checked.
Depends. On the stenosis on the other side. The one completely blocked can't be fixed normally. Keep eye on other side with yearly ultrasounds. Scan earlier of you are having symptoms. Probably need to be on aspirin.
Look - opposite side. A carotid artery that is 100% blocked ( chronically ) is not treated. Rather careful monitoring of the opposite carotid is done & when significantly narrowed ( > 60% ) or causing symptoms from material going downstream ( TIA or stroke ), it should be treated. Open surgery is the gold standard and needs to be done by a surgeon with experience and low complications (stroke, heart attack, bleeding).