[Ozuma-Rii]

Maguskn, if you have an exam, then go for it, after all real life is more important...

Ok, I will rebut whatever I can right now on the top of my head with a handphone keyboard.

First, about the many "no evidences" that you were Ranka knew that she could be the ECM if she sung, so she She was also with, with your own words, "super haxx ninja Brera", thus it is effectively double assurance that they would succeed. She had no evidence that Grace would show up, no evidence that Grace knew what her song was doing and how to even control her, also, Ranka is shouldering all the responsibility for the Vajura and Frontier, which would escalate to the whole galaxy in due time, unmentioned. She realized she could not take the responsibility that frontier entrusted to her, that way, so she took on the responsibility another way, in her own way.

And the point about Ranka misinterpreting the embrace--at that point of time, she was about to confess to Alto. The immediate first impression of the embrace with someone else other than her. No sane person would go calmly and ask Alto "Hey, what happened?". It is also noted that Sheryl did not even bother to explain to Ranka, merely slapping her and telling her to be professional even when she was emotionally upset.

And about the point that she was easily brainwashed and Mind-controlled: I am going to stop it so hard that even if it were Brera, it will still never rise again.
Let me drive the point home: Ranka was being emotionally traumatized by Grace. She was shown her past. She was still in shock. She was being weighed down by a myriad of things . This allowed Grace to exploit her emotional weakness. Ranka was, I repeat, in no condition to fight back, seeing the "sins" committed and what her songs "did". She has no evidence as to what her songs did other than confuse, and perhaps calm the Vajura. She had no evidence to her past. As I know, humans have a self-protecting system that deals away emotional trauma that one cannot accept, causing selective amnesia. Thus, there is no position to argue that Ranka could have fought back against Grace, seeing that she is not the fighter type (I mean physically fighting) nor is she some sort of iron-willed Messiah, capable of laying down her emotional trauma like nobody's business.That is what makes Ranka human and relatable.

And, what do you mean by Ranka giving up Alto to be graceful.
I would already give her a medal of honor for actually going on a risky mission (actually it is not that crapshoot. You are talking about a ECM and the best pilot here). She already redeemed her mistake in episode 20, IF you consider it a mistake.
She has done all she possibly could. The part where you said that she forgot to tell everyone about the visions? Let's just say that you are putting a 16 year old with the personality of a 12 year old into a horror situation. The trauma is enough to put your point to rest.
And about the part about her changing songs--she doubted the new song to begin with, and only because she is still a "kid" and she still sought opinion and approval. With encouragement she naturally changed her song like an adult tells a child to do something.
There.