Complex sentence: Because of the recent contretemps over van Dieren, Heseltine submitted these pieces as "Peter Warlock"; they were published under this pseudonym as was all Heseletine's subsequent musical output, his own name being reserved for critical and analytical writings.
Split into simpler sentences: Because of the recent contretemps over van Dieren, Heseltine submitted these pieces as "Peter Warlock". They were published under this pseudonym, which he thereafter adopted for all his subsequent musical output, reserving his own name for critical and analytical writings.