And Fred is not really happy to be with the Slurpsons. Forced smile and tired face.
he put himself - alone - in this mess.

Yes he did - and he was very starry eyed about her at their wedding - so I think it was his choice.
But - for reasons unknown to me the senior Danish Royals don't marry people from Denmark - Queen married Henri from France, her sisters married a German and a Greek, Jokka married Alex (Euro/Chinese) from HK, and then Marie from France. And then of course Mary - Oz from Scottish immigrants . . . there are the stories that he wanted to marry a Dane but got told no . . . so maybe he settled for Mary . . .
IIRC, it’s not any kind of law but is related to the size and population of Denmark. Geographically and population was, it’s small. There is a great deal of knowledge that is available about Danes, not even “six degrees of separation,” but more like two or three. Thus, any potential royal marry-in from Denmark would be fairly well known within a short period of time and whatever their background is, would be exposed in short order. Better, in Danish Royal Tradition, to have a bit of mystery or veil by marrying from outside.
Now of course, with social media and Google and our sleuthing here, it’s much much easier to uncover the background of any candidate for the royal family.
But yes, Fred was deeply in love with and was dating a Dane at the time that he met Mares. Between the Queen’s frowning on marrying a Dane and Mary’s ability to clutch/stalk like a pro, that existing/overlapping relationship. (Bettina) fell away and there was La Donaldson, in all her glory, ready to swan forward.
Fred’s tears at his wedding were certainly not of unbridled joy, but the frustration of the trap closing behind him. He looked hung over and slovenly, and Mary was an utter bitch.
So really, nothing’s changed. Begin as you mean to proceed, by golly.