This part of the saga began to write itself before the missing pieces between 1925 and 1969 could be filled in.  That was because there were many pieces of the Leviathan, Parallel Time, pre-1840, and 1840 sequence I didn't care for.  

By the time Quentin returned to Collinwood on the series Dark Shadows the writers didn't seem to know what to do with him and so he was mostly standing around, looking pretty.  It didn't seem plausible that someone with as much knowledge as he had about the occult would be so ineffective during these storylines and so I fiddled with the storyline a bit.

While I was doing that, another question occurred to me ... a what if question.  The first one threw the saga into a parallel time situation because something crucial in the storyline changed...