My quibble with the dictum ("that no composer can be considered among the "greatest" who did not write symphonies (with the sole exception of Wagner, who is great by dint of his SYMPHONIC overtures and interludes, not so much for pushing the envelope on belting out a "song", and not at all for his exasperating libretti).")is two-fold. With Mr.X we have a genius whose works continue to influence us today. The classical period ended with Beethoven and one might argue the same for the romantic period ending with Mr. X. His experiments in harmony and tonality open the door for expressionism, for Debussy, late Brahms, Faure, Tchaichovsky and others up to the present day. So he wasn't a great orchestrator. He obviously found his 'true voice' and his expression of that voice revolutionized not just that instrument but the greater genre. To some extent less is more. Why mess around with many instruments when you are making one or two do the work that others struggle to make 60 or more do half as well? Who knows what might have been had he tired of his 'voice' and decided to devote more of his energy to composing for larger groups? Can you really argue the talent wasn't there?
My second quibble is if you include Wagner on your stated criteria shouldn't R. Strauss make the grade as well?
"I adore art...when I am alone with my notes, my heart pounds and the tears stream from my eyes, and my emotion and my joys are too much to bear"-Giuseppe Verdi