In this composer's late Mass in E-flat major, the first eight bars of the Sanctus shift to B minor to G minor to E-flat minor. His longest symphony uses the three-key exposition "C major, E minor, G major," and two unison horns open it with the motto (read slowly) "long C, D, E, long A, short B, C." The 3/4-time first movement of a B minor symphony by this composer opens with an 8-bar introduction in the (*) basses and cellos. Schumann praised the "heavenly length" of this composer's last symphony after Felix Mendelssohn premiered it after his death. An entr'acte (ON-tract) from this composer's incidental music to Rosamunde (ro-zuh-MOON-duh) may have been intended as the finale of that two-movement symphony. For 10 points, name this composer of the "Great" C major and "Unfinished" symphonies.