This is not a marriage nor do you mention the conditions. It is not relevant
I didn't list the conditions because I don't know them, but it displays the difficulty (marriage or no) for converting from Catholicism to Orthodoxy.
Which prves that the Catholic Church permitted mixed marriages. The Orthodox Church does as well- see Nicholas' uncles Sergei and Vladimir. Both of their wives where Lutheran and converted much latter. It is customary for at least one partner to be Orthodox- there are execptions that aren't important here
Valena answered this.
The myth of Helene and Nicholas has been dealt with. If she was renouncing her Catholicism she was doing it on her own. If she obeyed her father's wishes, it was him and not the pope who blocked her marriage to Eddy
That was for Eddy. She was willing to go to Rome and see the pope in person to try and persuade him. Her dad (abusive jerk that he was) told her no. And got the pope to do his dirty work for him and tell her no (not that the pope would've agreed to it anyway). With the Russian match her dad was again against it, since she would've had to convert. If she
had converted without the pope's approval, it would've been a shortcut to being excommunicated, which would've meant that no one in her family (parents, siblings, cousins etc) was theoretically allowed to have anything to do with her, they were to act as though she was dead. And in the 19th century, religion was a much bigger thing than it is nowadays.
Your fina example only serves to prove that the formula existed and the rules known. The Austrians were sticklers for the rules and the rules would preclude an ORthodox to be buried in a Catholic cemetery- just like the rules forbade her from taking communion
The fact that it only happened
once is indicative of how
difficult that formula was to apply. And even then, it's a difference between a grand duchess marrying
out of Russia to be consort, than a princess marrying
into Russia to be tsarina. Olga Nikolaievna, Yelizaveta Mikhailovna and Elena Vladmirovna were also requested by Catholic suitors (the Archduke Stefan of Austria, King Maximilian I of Bavaria; Henri, Comte de Chambord; and Albert I of Belgium) and considering that even the tsar was willing to sign off on those matches (I'm not sure about the Elena-Albert one, though), shows that they had no problem marrying a grand duchess to a Catholic, they had a problem with a tsarevich marrying a Catholic.
Another example of the non-dispensation to convert is about Ekaterina II's planned marriage for GD Konstantin to a daughter of King Ferdinando IV of Naples. The pope refused to play along (IIRC) although her parents and Ekaterina were both for it. So, IMHO, the pope seems to be the common factor in most of these situations.