Complex sentence: Those lyceum members advocating an independent country were mainly British, including Dr. McLoughlin and his HBC employees, although many former fur trappers (predominately French-Canadian Roman Catholics) and the region's Jesuit missionaries sided with McLoughlin on this issue.
Split into simpler sentences: Those lyceum members advocating an independent country were mainly British, including Dr. McLoughlin and his HBC employees. Although many former fur trappers (predominately French-Canadian Roman Catholics) and the region's Jesuit missionaries sided with McLoughlin on this issue.