I‘m sad about Lena, too because she always seemed to love skating so much. And I think she wants to continue but back injuries are tricky and dangerous. I think she should heal completely before trying to compete at Nationals. Otherwise she risks her health. What use does it have if she isn‘t at her best and ends up somewhere distant from the top? It would probably crush her self confidence even more.
And if I‘m completely honest: I like both Sima and Lena so it‘s not out of pure bias but this season Sima deserves to be at Nationals more. Yes, Lena was injured and it obviously isn‘t her fault at all but she barely performed this year and Sima went from competition to competition, trying her best to get a spot. So I think it would be fair to remove her from the list of participants now and put Sima in, giving her the chance to prepare for the competition. It would be better for Lena and her health (I think she still needs some competing before jumping right in at the hardest competition possible, Russian Nationals) and imo, it would be what‘s fair.