The mysterious absences could be explained in part by the family's having two residences (Brunswick Villa, Southport and Fairhurst Hall, Wrightington) Ellen tombstone states that she was of... (both addresses) the name of the villa is clear on the tombstone.
As they were wealthy, they could afford to go abroad, for their health and might just miss the census. An alternative explanation may be a mistranscription of name(s)