Complex sentence: Gschladt was very close to Leopold Mozart who wrote a Serenade specially to be performed only by him, and when Gschladt was unavailable, Mozart preferred using a viola soloist over another trombonist.
Split into simpler sentences: Gschladt was very close to Leopold Mozart who even wrote a Serenade for Gschladt. When Gschladt was unavailable, Leopold preferred using a viola soloist over another trombonist.