I have a strange feeling that the Dowager Empress was so possessive of her son (and especially after Alexander III's death) that she would have disliked any wife of his (maybe with the exception of cousin like, for instance, Princess Maria of Greece). One of the reasons why I've always been interested in Nicky and Alix, is the fact that they actually married for love, which was a rare thing back then.
Queen Alexandra's great possesiveness towards her
children is probably true of her sister Marie as well, and any daughter-in-law would probably have had difficulties - Queen Mary had a very trying time of it once Alexandra realised she wasn't going to be the first consideration of her dearest Georgie, and Mary was generally approved of in advance. Whatever the Empress may have believed, I don't think Marie of Greece or even Helene of Orleans would really have stayed quite as popular with their mother-in-law unless they were prepared to be very self-effacing indeed.
Marrying for love amongst heirs to the throne was unusual, but not completely unknown - Franz Josef of Austria married for love, as did Wilhelm II of Germany - and indeed, the Prussian royal family, with the exception of his parents, were strongly hostile to the latter's proposed bride and it took a great deal of effort for them to accept the match.