1) Yes it is a sin—intellectually speaking—for someone to violate it. That means if it turns out on the Day of Judgment that it was haram or wajeb, and one didn’t follow it, then one won’t have a valid excuse. Yes, in obligatory precautions you are allowed to refer to the next most knowledgeable scholar.
2) Yes everything is halal until proven haram. However, sometimes a Marja’ may find evidence for something to be haram (but let’s say there is also evidence for it to be halal, or there is a reason why he doesn’t issue a full fatwa that it’s haram) then he will say as an obligatory precaution it’s haram. This means his follower must avoid it. If he didn’t and it turned out to be haram he would be held accountable.