There is also a difference if this was a Law from the cvountry like in Romani or a House Law like it in the case for msot of the former reigning german Houses. The later of course easier tochange. And in the case of the bavarian marriages which where approved as dynastic in 1999 it was not only Duke Franz alone but a Family Council who decided about this.
In the past decades we have seen many former reigning dynasties alter laws or rules surrounding marriage and succession.
In Romania the current pretender is styled by the Romanian government as HM Margarita curatrix of the crown. In spite of the fact that under the last royal constitution of that country women had no succession rights.
In Bavaria the current Duke of Bavaria has retro actively turned marriages from some of his cousins from non-dynastic to dynastic so that in the long run the succession after him and his brother will go to one of those lines. In spite of the fact that there are other lines who did follow the guidelines from before the fall of the monarchy.
In Austria and Hungary the last crown prince Otto changed marriage laws of his house. Going from a short list of acceptable families to allowing all christian brides in the family and making those marriages dynastic.
In France both the father and grandfather of the current count of Paris have singlehandedly altered the line of succession to suite their needs and convictions. Fortunately that has no real impact unless the lines of Jean and his brother Eudes were to end.
Those supporting Luis-Alfonso de Borbon as pretender to the French crown have to go even further in destorting history to get him to be the pretender but that does not mean it doesn't happen.
In Saxony the succession was also altered to provide heirs and in two other houses Salic law has been replaced by female lines of succession due to the lack of male heirs.
All these examples show that monarchies are not frozen in time but various dynasties or at least their Heads have chosen to adapt some rules to fit in with modern times.
In those houses the de Jure monarch has has acted and it has been accepted. I see no reason why it would be acceptable for those houses but not for the Russian Imperial family. Of course it would have been a lot easier if Georgy had found his true match in Theodora of Greece or Altinai of Montenegro. Still that does not mean that his mother as the de jure Empress of all the Russians cannot adapt rules or simply follow Nicolas I's lead and declare a marriage equal and as such dynastic.
If one Tsar can install the fundamental laws any of his successors can alter them. Leaving Maria Wlvadimirovna with the option to accept the marriage of her son and heir to Viktoria Romanovna.
This is true only if the successors are ENTHRONED sovereigns, which is certainly not the case with Maria. The fact is that the Romanovs have not reigned since 1917, which means that there is no choice but to abide by the laws existing as of then.
The whole point of succession and dynastic membership in non-reigning houses is HISTORICAL representation. The fact of the matter is that one simply cannot claim this, on the basis of attempted alterations of laws long after the said houses have been deposed.
It's worlds different from enthroned dynasties, which can legally and legitimately change their house laws.