The Constable should have asked for a more explicit promise, then.
Exiles without land or perspectives can't be choosers.
Also, couldn't the Constable have married some other woman?
Technically, he could have. But again, exile without hope of return and unfit ambitions are a bad mix.
Houses such as d'Albret and La Tremoille?
Yes.
Foix and Albret, essentially.
In addition, off-topic, but Charles III, Duke of Bourbon only got stripped of his lands because Francis I's mother could claim a genealogical relation to a previous Duke of Bourbon, no?
The Parliament of Paris decided that Suzanne de Bourbon was the tenent of the titles and lands, of her father, which Charles more or less took on himself due to being descendant of Jean I.
Eventually, Louise de Savoie clamied these due to being the grand-daughter of Charles I, and the main reason why the Parliament agreed with her was because the king willed it so, while his mother's rights weren't really that strong.
If so, could a similar genealogical relation have been found to strip the Duchy of Brittany from a surviving House of Dreux in this TL?
Brittany still going to be a geostrategical matter ITTL, it's likely you'd have dynastical ties between Valois and Dreux IMO. But altough these might possibly not be enough legally for a captation, it's not like Valois didn't know how to shenanigan their way with dubious claims or debatable legal points.
Again, nothing certain, but a risk to keep in mind.