Hmm. Not really if the writers want to create tension and anticipation for her first singing appearance. And when we/they hear her sing she is so good that they are blown away and feel that they can't compete, circa Showmance and Vocal Adrenaline. It would take that tension/anticipation completely away if she did it in the episode before. Now granted a lot of people know who Jessica Sanchez is already but there are still some who don't watch Idol. And in the end it doesn't matter because it is about the narrative and the characters experience of her singing. And her awesome talent. And to be honest I don't even know if Lea Michele/Rachel Berry could beat Jessica Sanchez hands down when it comes to singing.
As far as Rachel not getting a solo and Cassandra, well sure it might be disapointing to some but Kate Hudson is an oscar nominated movie star and they aren't going to waste that opportunity. And quite frankly they have to cater to her somewhat. You could honestly say it is ridiculous not to have Matt sing a solo or Naya over Cassandra. But this is a pure professional business decision. For whatever reason RIB seem more content to give most of the music to the Lima side again which leads me to believe yet again that this rumour of no Lima next year a crock.
Plus considering that hardly any of the people singing solos this episode with the exception of Jake had much of any singing all year, I am going to keep my mouth shut in terms of complaining because most of the people singing were way overdue to sing a solo anyway or at all for that matter.
I wouldn't have an issue with any in the regular cast singing in place of Cassandra. If they want to give something to people who haven't sung much this year, that's whatever. But if 1 person was going to sing in a tribute episode, that's also the penultimate episode of the season, regarding the Rachel/Cassandra story, I think it should have been Rachel.