What's that got to do with anything? FTR: no, I don't believe that every decision SCOTUS makes is necessarily constitutional. But they're still the law. You do believe in the law, don't you? Dred Scott v. Sandford was one of the poorest decisions ever handed down, but it was still law. Roe v. Wade is crap but we're stuck with it until we can build a super-majority.
Yes and no. It is treated as such and many accept it as such or are forced to accept it as such. However, when the Federal government acts outside the bounds set for it by the constitution, it is acting illegally itself, and thus such laws are technically invalid.