AFRANK: It is rigid if you mean you might move it.
That is why I said to use care. Take your time. You will not screw up anything if you are gentle and take your time. Again, the mirror is rigid and fits into a slot. As far as removing the reflective silver surface: mere water with a tiny bit of soap will not ruin the silver surface of the mirror. Alcohol or, perhaps, lighter fluid, will.
Lukas_87 is correct in that removal of the silver would remove the shadow image which confirms focus. - David Lyga
I used a non alcohol liquid used to clean the digital sensors of cameras. I wasn't sure if it had alcohol and all it says is that us made of bio materials :/. The cleaning went well and its more clear now, but was worried I had damaged something I wasn't able to detect. The only issue i have is with the viewfinder frame. It seems to be tipped on one side. instead of looking like this: http://www.flickr.com/photos/mnmleung/7320083890/
it seems like crooked frame on a wall, tipped to one side. It was like that before but I could not fix it :/