Titles are darn confusing. I would say this is easily the biggest mistake I see in historical fiction manuscripts. I occasionally catch an error in print!
Instead of getting into the nitty-gritty of why a Countess is married to an Earl, let's think of the big picture. There is a pattern here. Can you see it?
Example names:
John Smith, Title of Grand and Jane Smith, Title of Grand
Royalty
King John
Queen Jane
Prince John
Princess Jane
Hm, all first names only. In fact, the King and Queen might not need even their first names to be identified. These are powerful people.
Peers (roughly in order of precedence)
Lord John: son of a duke
Lady Jane: daughter of a duke or earl
Dukes were often cousins of the king. As such they also go by their first names. This is a title that can only be conferred by birth and can't be lost. When a daughter of a duke marries she keeps her title, unless she snags a prince and then she "levels up."
Lord Grand: A Peer who holds a title, or the eldest son of a duke, marquess, or earl
Lady Grand: A woman married to someone who holds a title
The eldest son of the current lord would use a courtesy title (generally a title one or two ranks below that his family had before earning the current title.) Courtesy titles might extend to the eldest son's eldest son, but never applied to nephews or brothers who were in line to inherit.
Lord John Smith: “spare” sons of a marquess
Lady Jane Smith: daughter of a marquess
Lady John Smith: the wife of Lord John Smith (this may be accurate, but your readers will hate it)
The Honorable Mr. John Smith: son of a baron or the “spare” son of an earl or viscount
The Honorable Miss Jane Smith: daughter of viscount or baron
You can abbreviate this as "The Hon. Mr. John Smith." You only need to use his full title in formal settings, like when he is being introduced.
Gentry
Sir John Smith: Knight or baronet
Lady Smith: wife of a baronet or knight
Knights don't have a family seat, so they have no title to share with their lady. Instead she gets his family name. And yes, in modern times there are female knights called Dames.