I’ve also heard people speculate a part of the reason why she never remarried was that she thought she was infertile (I do doubt that, but she turned down at least two marriages OTL) so maybe with the knowledge she’s not, she agrees to remarry here?
The other theory is that it was a Konstantin Pavlovich-Anna Feodorovna type marriage, and Marie had no desire to go through that again. Not to mention, how would she
know she's infertile? Amalie of Oldenburg had Mullerian agnesis and yet no one thought she wouldn't have children (in the 19th century); Maria Teresa d'Este, comtesse de Chambord, likewise couldn't have had kids, and I don't see Madame Royal picking her if that fact had been known (it's only with medical developments that we now know she was).
And if it
was known that Marie was infertile those two marriage offers (François I and James V?) wouldn't have been made. It strikes me as a retroactive attempt to explain why Karl V didn't just
force her to marry (after all, he did it with Eleonore-François and Kristina-Sforza).
if I’m remembering correctly she basically got stuck in the Netherlands because her aunt died and no one else could do the job
She left for the Netherlands because she knew Margarethe would back her choice to
not remarry. After all, Marge herself had done it. And if there was
one person whose "authority" Karl respected, it was Margarethe's.
Or at least that she remains in Hungary, maybe even doing so to demand it as her daughter’s inheritance?
She remains in Hungary,
@Jan Olbracht and
@Zygmunt Stary can correct me, but she'd wind up married to Zapolya (in all likelihood) and her daughter was heiress to absolutely
nothing due to the fact that Hungary was elective. And the magnates wouldn't elect a princess who's just going to put them into personal union with PLC (the Hungarians weren't crazy about that union last time around) or the HRE.
An interesting POD woudl be Marie staying in Hungary, marrying Janos Zapolya but having no kids. Zapolya still makes his deal of OTL with François I and promises to install one of François' sons as heir to Hungary. Marie's daughter winds up married to Charles d'Angoulême. Voila, Valois Hungary.
Yes, I know it's borderline ASB, but it would be fun to see the sheer chaos.