Complex sentence: He also was careful to avoid spending six continuous months in Pennsylvania himself (which might have been interpreted as his establishing legal residency), arguing that he remained a citizen of Virginia, and subject to its laws regarding slavery.
Split into simpler sentences: He also was careful to avoid spending six continuous months in Pennsylvania himself (which might have been interpreted as his establishing legal residency). He continued to say as he was a citizen of Virginia, he was subject to its laws regarding slavery.