Marguerite is headstrong and bratty, but I see nothing wrong in her desire.
Sure, she needs to learn, and her kind's society shuns her choice... but it is still her choice, if a race just stays the same, they are fated to meet their end, Marguerite and her uncle are necessary for the continuity of their collective spirit.
I understand why Graham acted like that... but I don't agree with his choice, if he is worried about the hardships she will meet he should be there, guiding her, but admonishing her just because she wants to make her own life, I do agree on calling her out on her brashness but he was exactly much more mature by just letting her getting hurt like that.
The elves here may be putting the cart before their horses, by diluting their ego they aren't really becoming one with nature, they are just turning their backs on life... sure, I have no doubt there is more to their ideology, but they are in a collective state of inflexibility that could lead them to a terrible fate.