My son just had this problem, he tried penetrating oil, double plyers ext. short of ruining the stems and I lent him my Dremel tool with the diamond metal wheel. Told him to make a shallow cut or two like you said and use a thin screw driver to pry it open and that worked, just don't go too deep into the thread but a slim cut into the threads wouldn't matter. Never use metal valve caps that are a dissimilar metal they will corrode.
