Complex sentence: By far his favorite living composer was Jean Sibelius, whom he championed throughout his career, and he exchanged letters with the composer; in addition Finland awarded him honors and invited him to Finland to speak to honor Sibelius's 75th birthday.
Split into simpler sentences: By far his favorite living composer was Jean Sibelius, whom he championed throughout his career. Late in Sibelius's life, Finland awarded Downes honors and invited him to Finland to speak in honor of Sibelius's 75th birthday.