To me, the importance of raising the arms to 5th is not in the situation in which the ballerina is in balance and the raising of the arms might throw her off, but rather in the situation in which a ballerina is somewhat off balance from the moment she releases the cavalier's hand. The time to grab the next cavalier's hand is far shorter without going to 5th. Indeed, I think one could never be in balance and still grab that next hand if it were placed close enough. I have seen the next cavalier place his hand so close to the current cavalier's hand that the ballerina only has to move her hand 6 inches to grab it. The power of going to 5th, is that all such "safety valves" have to be foregone.
I think it is probably a "mental" thing caused by nervousness. Granted I tested this without an audience and without being on pointe, but I just got into tree pose, found my balance and giving my hand to 10 or 20 or 30 cavaliers by simply moving my hand to the next imaginary one or raising my arms above my head in 5th and bringing them down each time did not make any difference at all for me. Of course, I was on a flat foot. But ballerinas should be so used to being on pointe that I would think that they can easily find their balance and stay on pointe the way I can stay on one flat foot. I will say that I am sure that being on pointe and staying there is much harder than tree pose in yoga. I really do think any wobbling or fear, however, is actually caused in their minds by an expectant audience sitting there watching them. If you are balanced (found your center) and good at balancing (and all ballerinas are probably much better at finding their balance than I am), this is really smoke and mirrors. It looks incredibly hard to the audience but it really isn't to a dancer (but maybe dancers could chime in here if they have something to say), I suspect EXCEPT for their fear of messing up in front of an audience and the fear that one of the cavaliers might move their hand as she takes it and throw her off accidentally. So what I am saying is that, yes, anything can throw someone off balance (thoughts, the cavalier's hand moving, stage fright, etc), but just by itself the crowning of the head does not make the move/pose harder except for a ballerina who is freaking out about her balance already. I think the fact it is such an exposed moment. It is a static moment for the ballerina, so moving around the stage can not hide a slight mistake. She is standing still and balancing and just moving her arm.....very exposed during a very iconic moment in the ballet, and that creates nerves, b/c, yes, people can lose balance when they least expect it, but I have come to believe it is a mental thing. I don't believe moving the arm up into a crown messes the ballerina up. I think her own thoughts or fears causes it, and if she falls out putting her arms into a crown, she would also fall out simply passing her arms from one cavalier to the next because the state of mind she is in.
I think staying balanced is a weird thing. It is very mental and it is less about strength and much more about being in an almost zen moment and feeling your body find its center of gravity and just mentally staying in that place in your mind. I think I watched a video once and it showed all these Royal Ballet dancers doing zany things while balancing on one leg like drinking tea while staying balanced. They were moving their arms in many different ways. They were all rock solid as they moved their arms all over the place. However, put them in front of an audience where there are no re-takes and make it one of the "important" moments of the ballet, and THAT is what plays with their mind and causes them to worry about falling out. Not where their arms are moving.
I have given advice to people who constantly fell out of tree pose during yoga class, and I told them to stop trying to use the strength of their leg, grow tall and reach the top of their head to the sky and somehow find your center and suddenly balancing becomes incredibly easy. It is actually more about "letting go" than muscling into and holding the balance as weird as that sounds. Hard to describe, but once you find your balance it feels like nothing can throw you off except someone pushing you down. Hands and arms can move all over the place in every direction if they are well balanced.