#3) Does it matter that the parents understand the situation for which kid moved grades? When I am asked now, I just say the school decided that this was the best arrangement for now and then I shrug my shoulders and change the topic.

#1) Your daughter will probably find that some kids are like these ones at the concert and some will be so excited to see her. Kids move in and out of friendships all throughout school. It can be as simple as not being in the same homeroom that causes these shifts. Just continue to encourage her to socialize at school and school events and activities.