Well, Xzenia and Sandro's marriage was perhaps happy by royal standards, if not by those of the average individual. They were very much in love when they married, more so than most royals, they had many children, all sons execpt for one. This would make the marriage a success by both royal and non royal standards, except for later things. They did later have a marriage that fell apart, but perhaps always retained some affectionate memories of what it had been, once. At least, I think they did even if they no longer felt that way.
As for Olga, she said her first marriage was forced upon her. Yet, it might have been desirable to her to escape from her mother, and yet with this marriage she could stay in Russia and have a comfortable life. So, it might have been okay with her, even if it wasn't her choice. Her mother certainly liked to seem to indicate that it was her choice, even if it was not. But, then, she accused her mother of plotting it, so her mother may not have wanted to admit it. I think her mother planned it, but Olga may have been more okay with it than she ever wanted to admit later. Prince Peter did get advantages from her, in being married to her. Olga did later know what she wanted, but she was young and may have been confused at the time of her first marriage. She may have tried to see the good in it rather than the bad, until she fell in love with Kulikovsky. Then, and perhaps only then, did she realize what she had been missing.