As for the rudeness, LW is essentially eating two courses when everyone else eats one. Even if she orders them at the same time, chances are that it will take her longer to finish eating. It's not particularly conscientious to waylay a tableful of diners who are pressed for time. (If she is finishing at the same time, I wonder whether she is able to participate fully in the conversation given the extra food she's eating.)
Well, I'm assuming that at least some people will go on to order dessert after the mains so I don't think time is an issue.
Maybe it's different in the US, but at most restaurant meals I've enjoyed, in both business and social settings, the party did not make a decision on whether to eat dessert or not until after everyone at the table had finished their mains. Unless the LW is ordering the dessert in place of a starter, this would almost certainly be disruptive to everyone else's dining experience. If she received both courses at once, the rest of the table would either have to wait for her to finish her second course (the main) before asking for the dessert menu, or otherwise interrupt her meal to do it.
It would just throw off the tempo of the situation.
Also, as others have said, her behaviour and especially her explanation for it (rebelling against childhood parenting?
) make her seem childish, attention seeking and snowflakey.