Salyu doesn't need a backup singers to sing "tough" parts, that's ridiculous. You should watch the whole concert next time instead of just check out one live cover. 心 live is proof she has fantastic vocal talent.
Also I don't understand why you would think Salyu would change "Calling" THAT much since it was just an extra song to add to the concert. If she wanted to take the time to give it a unique twist, she would've gone in the studio and added it to the album. It was simply just another song for the concert that fit the theme very well, so I think you're overreacting by calling it "silly" she didn't do anything with it. Why should she? The whole s(o)un(d)beams project is original enough as it is.
Wow, b-l-ackstage. I consider that live performance of Calling as one of the highlights of the concert. Actually I find it more appealing than the original. I don't understand the 'backup singer' thing, since I think the most interesting part about this version (and the whole salyu × salyu thing) is the experimentation with vocal layers and patterns, mostly in a minimalistic way. Well, now that I think of it, actually this song is quite the exception, it relies strongly on Salyu's versatility.