Complex sentence: As British monarch, Edward was the nominal head of the Church of England, which did not allow divorced people to remarry if their ex-spouses were still alive; so it was widely believed that Edward could not marry Mrs Simpson and remain on the throne.
Split into simpler sentences: As British monarch, Edward was the nominal head of the Church of England, which did not allow divorced people to remarry if the ex-spouses were still alive. So it was widely believed that Edward could not marry Mrs. Simpson, and would remain on the throne.