"For from her, all good descends"

“Lady, in truth, she is so courteous, so beautiful, and so wise that God made no law and no language in whose spheres one would find such a graceful lady. Ever since God formed the first woman out of Adam’s rib, there has not been so renowned a lady. And she deserves to be called thus, for in the same manner as the wise masters educate the little children, my lady the queen educates and instructs the whole world. Of her springs all good: It flows from her and begins in her. No one could leave my lady and go away despondent. For she knows well what everybody wants and what she has to do to please every single one of them. No man can do good or win respect who has not learnt of my lady how to do so. Never was a man so out of sorts that he parted from my lady but not from his worries”

Fast forward today, I do notice some kind of undercurrent with Gwen and the various groups that she purportedly hails from across the tradition:

(Source: Arthur in Medieval Welsh Literature, by Oliver Padel)

(Source: Historia Regum Britanniae, by Geoffrey of Monmouth)

(Source: Perlesvaus)

(Source: Sir Launfal, by Thomas Chestre)

(Source: Les Enfances Gauvain (Fragmentary))

From the Welsh Myths' claim of Guinevere as a giant's daughter, to the various attributed nationalities, and even having pagan kinsmen...

Even when the tradition has settled on a defined (fictitious) country as her birthplace, there's still some those little background quirks that imply some points of dissimilarity from the ways of Arthur's lands:

(Source: Segurant Knight of the Dragon)

(Source: Idylls of the King, by Alfred Tennyson)

[...]

[...]

[..]

(Source: Culhwch and Olwen) (Link to Translation)

The regretfully brief and anachronistic adventures of William the Bastard as Knight of the Round Table, who met his end at the hand (hoof?) of the Great Boar.

(Source: Sir Gawain and the Green Knight)

It has always been joked around about how Morgan le Fay, the legendary dark sorceress and mistress of all magic etc., would resort to such a weird and goofy way to stun the Round Table and kill Guinevere.

But I think funniest aspect of it all is not in how Morgan decided that a glorified halloween costume and going "boo!" on the Queen was her preferred method of assassination but that, in another story, this should have worked.

The thing about Medieval Courtly Romances is that a major thematic device in the genre is overwhelming emotions. Characters in these sorts of stories are often gripped by intense reactions and exaggerated passions that drive much of the drama. That's why people faint a lot in these stories, why knights go mad and run into the woods when their LIs reject them, why dying of broken heart is a real threat - just ask Iseult and Galehaut. Hell, Guinevere literally died of broken heart in Tavola Ritonda.

Guinevere should have died here as well. The Green Knight should have been so shocking that it leave you white as a ghost. Morgan's plan should have worked. And it doesn't. Likely because SGatGK was not that kind of story anymore. It's an English romance starring Sir Gawain, written in the Late Medieval period, long after the old tropes of romances have become cliché (if they were ever popular with the English to begin with).

Since the love story of Tristan and Iseult appears to be very, very archaic, with its roots in oral tradition, it follows that T&I probably has had many, many regional variants for how the plot goes. Maybe some versions don't have the love potion. Some end in a happily ever after, some do not.

It might be that, in some (possibly Breton) folk versions, Tristan just... fell out of love with Iseult (maybe it was ultimately just a fling in the end). Tristan then maybe starts to distance himself from Iseult or even tries finding a new LI. This angers Iseult into planning his death for scorning her, poisons him and, possibly, commits suicide right after. This would sound like a typical moralizing folk tale about reckless love and irresponsible persons.

The hypothesis, then, is that the popular version of Tristan and Iseult, with the two seemingly unrelated Iseults, is Romance writers trying to fold all those disparate folktales into a unified canon while also trying to have their cake and eat it - wanting a story where Tristan and Iseult remain in love, but also wanting to incorporate the tragic ending, rationalizing the Iseult who lets Tristan die as a separate person from the "true" Iseult.