I have no idea where you're getting number 2 from. If anything it was the other way around. Takayama Minami (Kanade) and Mizuki Nana (Tsubasa) both have rather deep singing voices. Through most of the concert song, any time they were singing in harmony, Mizuki Nana was overpowering her.
Overall, I think the two of them really complimented each other. Takayama Minami has a sharper voice, while Mizuki Nana's range generally overpowers and uses way more vibrato. They each covered the gaps of the other. It was a really good casting choice. Maybe second only to Horie Yui and Toyosaki Aki playing completely believable sounding twins in Otome Youkai Zakuro.
Now if it is just because you're a Mizuki Fanboy...never watch this.
The primary reason there was no emotional points for Kanade's death was we never really got to know her. That said, I liked her performer smile better than Tsubasa's.