In the USA, could a clause (article, proposition) of a State Constitution be held unconstitutional, with respect to the USA Constitution, by the Supreme Court?
Yes. The supremacy clause in the US Constitution means that State law is supervened by Federal law. This includes State constitutions.
If it could, has it ever happened?
Yes. Here is a list of all SCOTUS cases that have overturned state law. For state constitutional provisions see nos 37, 49, 50, 51, 54, 74, 75, 139, 140, 151, 182, 188, 202, 207, 224, etc. (there’s nearly 1,000 in the list, you can go through them yourself).
Also, could a decision by a State Supreme Court be reversed by the USA Supreme Court?
Most of those cases will have gone through the State Supreme Court first. Except where SCOTUS has original jurisdiction (those involving disputes between the states or disputes arising among ambassadors and other high-ranking ministers) the state courts must have ruled to enliven its appellate jurisdiction.