I'm in a piece this Saturday with 9 other adult dancers and the issue of being in sync is something we've been working very hard on. Unfortunately, the director/teacher/choreographer doesn't always notice when we're on time together but doing moves differently. So we go to each other and say we noticed a difference, work it out amongst ourselves best we can, then go to the director for clarification.
I don't know you or your dance or the person you're calling an "egotist." But I would encourage you to think of it in another way. This dancer may not be trying to upstage you, she may not even know she's doing it differently (even watching a video...I always watch myself, to find mistakes, and have to make myself look at it as a whole). She may not think she can do the moves as written and do them well (this may come from a place of fear of messing up). She may not understand that the dance will look better if you all do it the same (lack of a "corps mentality"can be deliberately refusing to work as a group, as you seem to suspect, or it could be lack of practice with it, lack of experience with the concept, no matter how obvious it may seem to you or even everyone else).
Can a couple of you approach her and say you noticed that the 8 of you are doing XYZ different from each other and let's figure out how to get everyone together. In my group, we often end up with more than one version (the director changed her mind at one point, which happens a lot, or the details for a particular move were vague and we all extrapolated). So we usually say both ways are good, we just want to know which one [director] wants. Occasionally, we will tell someone (or they'll tell me) that a move is incorrect (whoops, wrong leg or arm) but that's not common.
In your case, the moves may all be identical but there is a difference in how big or small a movement is. That's a legit issue for a choreographer to resolve. You can also ask the choreographer which way she prefers. Ask in front of the entire group, if you can do so in a neutral way that doesn't imply one way is wrong. Or pull the choreographer out before rehearsal and ask her to pay attention to that issue and to make a ruling one way or another to everyone.