I’ve been thinking a lot about Revocation!AU and how to organise the marriages so that it all makes sense. My thoughts:
Daena and Viserys had always assumed they would marry, and Baelon wanted that too. But the situation with the Velaryons was coming to a head, and Alyssanne wanted to arrange the marriages of Viserys/Aemma and Daena/Raymar.
Because: 1) Viserys desperately needs the support of the Vale and its knights; 2) Viserys and Daena’s marriage is certainly symbolic, but it will bring no support from the other kingdoms; 3) Alyssanne believes that Daena, with her wild temperament and love to swords, would be unhappy as a queen, whose primary duty is to bear heirs. 4) Alyssanne thinks that Viserys will not be able to cope with Daena’s temperament and will indulge her in everything.
To be fair, Alysanne had learnt from her mistakes with Viserra and had chosen Daena's fiancé with great care: Raymor is young, handsome, has a good reputation and a calm, strong-willed character; moreover, he is the heir to the second most influential House in the Vale. But Daena is resisting the marriage as best she can, whilst Jhaehaerys is wavering because Daena might set the Valе alight if she is forced into marriage.
And somewhere around this point, memories from another life come flooding back to Daena, and he/she decides to marry Raymar because 1) she/he desperately misses Jon and Rhaegar and wants them back; 2) after all, Daemon did not love Viserys romantically, and the thought of marrying him seems strange 3) Daena/Daemon harbours a subconscious fear that she/he will suffer the same fate as Aemma.
And so, Viserys is both hurt and upset, because he loves Daena, but he also fancies the sweet and calm Aemma. And to some extent, he feels relieved that he won’t have to deal with his sister’s fiery temper. Raymar is somewhat bewildered, for only recently his fiancée wouldn’t even look at him, yet now she has volunteered to marry him.
To be fair, it could also be as simple as Viserys (or Daena) deciding they don't want the match!
For your scenario, I think the main thing that doesn't quite click for me is doubling down on the Vale. Viserys/Aemma should be enough to secure Vale support, really. But also--dragons are nukes. Sending Daena away with her nuke (Caraxes) to marry a distant lord deprives Viserys of a dragon that he desperately needs, given that Balerion is dying (or possibly already dead, depending on when this is happening). Right now, the Velaryons have just one in Meleys.
It's sort of, which is more valuable: keeping a dragon close, or having Vale swords available? The fact that Daena has a dragon really changes the calculus IMO, and is a big reason why there would be pressure for a match between her and Viserys. Otherwise, you get a repeat of the Velaryons "having" a dragon from their Targaryen bride in Rhaenys, except it's whatever house Daena marries into.
I could almost see it being an uphill battle for Daena to get a match with Raymar.
(We've discussed before that Alysanne and/or Jaehaerys might have some concerns about a hot-headed Targaryen princess after what happened with Saera, and how that might lead to them seeking a "stablizing" match, but I hadn't really accounted for Caraxes at the time.)
The one workaround to all of this, of course, is that Daena claims Caraxes after her betrothal to Raymar, throwing everyone's well-laid plans into upheaval.
I do like the notion that Alysanne is worried that Daena's free spirit would be stifled. It's interesting to think about what she might have thought about Alyssa's suitability to be queen, since Alyssa died long before that became a possibility.
Then, of course, there's the Daemon memories wildcard of it all, aka when that kicks in and throws things off.