Gonna chime in with my two cents. In short, I'm in the "At this point, there's no reason to worry" camp.
Kalafina has had a very busy year. They've been travelling overseas, doing the Consolation tour, recording albums and singles non-stop. Add in the FJ work for Wakana and Keiko, and I'm sure it ain't easy.
Yes, at around the beginning of 2011, Wakana's voice--or, more likely, her singing style--changed. I think their performance of Magia at Lisani 2010 was the last time Wakana used her "old voice." Well, last official recording of it that I know of, anyway. There were some performances in 2011 that really worried me--Wakana sounding tired all the time, and some smaller issues with Keiko and Hikaru (but they're not severe enough to go into detail about). My worries pretty much culminated in the Vol.9 stream, where Wakana... really didn't sound good on songs that she'd previously performed (I guess because I had superior versions to compare them to).
However, several things made me believe Wakana isn't "losing her voice," she's adopting a new singing style.
1. Seeing Kalafina live at ACEN. I won't lie, I was both excited and worried to finally see them in person. I've heard so many phenomenal videos pre-2011, but everything since then has disappointed me a little. Was all the previous stuff just clever editing, or perhaps Wakana really lost her voice?
I came out of ACEN completely (and pretty much equally) impressed with all three girls. Yes, Wakana was using her "new" voice. But there's definitely aspects of her old voice present in many songs (Kagayaku, the "ahhs" in Signal, Hikari Furu). And while the tone was predominantly her new voice, it packed all the power I expected from the pre-2011 videos, and then some.
2. If we look through the Niconico lives, Wakana is sounding better with every one.
3. Regarding the beginning of Hikari Furu, I don't think it's strain on Wakana's part, so much as a) it's freaking tough to sing--a transition from a low note to a pretty high one, with no breath between them. Try singing it sometime ;) b) The way she sings that line is probably a stylistic choice. I had a "er... what?" reaction when I heard the first Niconico live, but after getting used to it, I think it's just how it's supposed to sound--the studio version is that way too.
4. She sounds excellent in the Christmas lives included with the Alleluia single. Pretty much like her old self, actually, except singing more softly to fit the arrangement.
Anyway, my conclusion: because of the new singing style, Wakana is less consistent than she was before. However, the Niconico lives show her improving, and she was truly amazing at ACEN. There are some worrying performances, I don't think sending a letter is needed at this point.
Disclaimer: I am a Wakana fan, so you can take my thoughts with a grain of salt. However, I am also a Wakana fan who greatly prefers her old voice and actually wants her to go back to it. While I can accept her new voice, I can be critical of it.