Not sure if it has changed, but in my experience No they are not required to attend all the practice sessions. Most years I went to Nationals the big names wouldn't show up until later on in the event. The ones who get there early and attend the practices are usually the lower ranked skaters at Sectionals that take in every tiny bit of their Nationals experience because they know it's the end of their season and may not even make it back again.
I have a question. Isn't there some rule that once you compete at Senior Worlds, you cannot return to Junior Worlds? So if Polina places fourth, she might get 4CC's but cannot do Jr.Worlds this year. Has that changed? Then again, if Mirai gets third and Polina is fourth I wouldn't be surprised if USFS chooses Polina for Sr.Worlds.