Indeed, using the strictest definition of commoner* even Prince William was a commoner until his wedding day when his grandmother make him a peer.
*Anyone who is neither the Sovereign, a peer, or the wife or widow of either.
In France, most titles could only be passed in male line (didn’t apply to the major ancient noble houses, whose titles could be inherited by females, cousins, etc…but AFAIK all such titles were eventually absorbed by the French crown), so they slowly go extinct.
Nobility, on the other hand, belonged to the whole family, so anybody descending in male line from a noble would be noble too, but this status was lost if the nobleman engaged in any non-noble occupation (essentially any job besides state or military officer), ensuring that impoverished minor branches would eventually lose this status.
Since nobility isn’t recognized anymore, there aren’t any noble, strictly speaking, in France. However, people with titles, and probably their relatives (although in fact such relatives are neither noble nor titled) would be considered noble in common parlance. For instance, I suppose that anybody called “de Rohan” or "de Clermont-Tonnerre " (very ancient French noble families) is probably socially considered as noble.
So’s Lady Edith. She’s a commoner who gets a courtesy title: a courtesy title is not a nobility title.
In Spanish, someone who’s a member of a noble family but himself not a noble would have been an hidalgo, shortening of hijo de algo, “son of something-or-other”. Since this didn’t just mean the children of the alta nobleza (the people with titles similar to those of the British peers) but the descendants of every knight and minor lord… well, one of the reasons the Don and Doña treatments got extended to everybody is that by now, the people who didn’t have the right of them by birth would actually be a minority.
So what about Lady Mary. She married the future Earl of Grantham and had a son. Her husband died before her father so he never actually inherited the title but his child will inherit the title upon the death of his grandfather. So Lady Mary’s Father is a non-commoner and her son will also be a non-commoner but she will be a commoner for the rest of her life?
Unless she marries a peer (or the monarch), yes.
She’s “Lady Mary” because she’s the daughter of an Earl; the facts of her parentage are never going to change and the associated courtesy title will always be available to her.
On marriage, if her husband had a higher status than her she would have acquired that status, but he didn’t; he was just Mr. Matthew Crawley, so she retained her status as the daughter of an Earl. Had he lived, and inherited the title, she would have become the Countess of Grantham, but he died of a virulent plot device, and so that never happened.
Her son in due course is expected to become Earl of Grantham, but that won’t affect her status. Similarly, had Matthew lived to inherit the title, his mother would have remained plain old Mrs Crawford.
To use real-world examples
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The 7th Duke of Marlborough, John Spencer-Churchill, had a younger son, called Randolph, who could be called “Lord Randolph Churchill” (he dropped the Spencer bit). Randolph had a son called Winston, who was known simply as Mr Winston Churchill until he was knighted in his own right and became Sir Winston Churchill
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The First Earl of Mornington had a younger son, who went by The Hon, Arthur Wesley, who changed the spelling of his surname to Wellesley, who ended up with lots of titles in hos own right, which ones is left as an exercise to the reader.
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The 6th Duke of Bedford had a younger son, called John. He was entitled to be called Lord John Russell (again Russell being the family name). He amongst other things became British Prime Minister and starved the Irish. He then was enobled in his own right, as the 1st Earl Russell. His grandson, the 3rd Earl, is known more by his own name than his title, another exercise for the reader.
And Lord John Russell is a particularly instructive example, because he sat in the House of Commons as MP for a variety of different constituencies between 1813 and 1861, which of course he couldn’t have done if he were a peer. It was only when he was ennobled as Earl Russell that he entered the House of Lords and lost his eligibility to sit in the Commons.
‘Lord’ being the general default address for peers, of any rank from whether Marquess or Baron * ( and found in other contexts unrelated to aristocracy ) it seems rather careless for them to have evolved it as a confusible courtesy title. You’d think it easier if a separate word had been adapted.
As a non-insulting example, as son of a peer little Johnny Russell above could have been called Hetman Russell ( or any other word ) unless he succeeded to the title or was ennobled in his own right.
I am aware Dukes may be addressed as ‘Duke’ directly, or ‘your grace’ just like there are a number of ways to address a bishop, but there are so few of them, Royal or Non-Royal, in the world most people aren’t going to run into one.
You’re missing the point, which is that the rules are supposed to be complex, arbitrary and confusing. That way they they help you to distinguish between those can navigate the rules with ease and so are familiar with, and comfortable in, this social milieu on the one hand, and the outsiders and parvenus on the other.
If the rules didn’t fulfil this function, you couldn’t tell whether they were One Of Us or not without asking them to serve you tea, and observing to see whether they put the milk in first.