A problem with her using the title of “Dutchess of Cornwall” is that Charles daughter in law inherits that title.
Actually, that’s not true.
If Charles predeceases his mother, William will not become Duke of Cornwall. That title is reserved solely for the oldest surviving son of the sovereign. However, if the next in line is not the oldest surviving son (as in this case) the Dukedom would merge with the crown until the Queen dies, whereupon William would become King and George would become Duke of Cornwall.
However, even if this were a normal dukedom, it would still not be a problem. The widow of a Duke is still styled as a Duchess, even if her daughter in law legally acquires the title. So, consider this scenario:
Prince Phillip dies and Charles inherits the title Duke of Edinburgh. Camilla, by marriage becomes the Duchess of Edinburgh (in addition to her other titles). If Charles then dies before ascending the throne, William becomes Duke of Edinburgh and Catherine becomes Duchess of Edinburgh. However, even so, Camilla could still be styled “Camilla, Duchess of Edinburgh” as a dowager duchess.
The Wolf