Thoughts on killing the mirror, the "30 foot radius," and evidence
1. Killing the mirror - I think it's very possible to do if you leave it covered. I don't think it's powerless in that state, but I would say it has less power. So if it was covered, I could see a hammer swing or dropping it off a cliff or something working decently. Maybe. And in a related note, I lol'ed so hard when the company bundled up the mirror to make it safe to move...if I was the girl I'd be like no worries, it's cool, just throw it in the truck any which way and let the road do its damage!
2. I saw some talk on the boards about the "30 foot radius." I don't think this is a permanent radius...it's just how strong the mirror is at the time she measures it in the movie. I think before that moment the radius would be smaller, and as the mirror grew stronger (like at the end of the movie when all sorts of shenanigans are going on) the radius would get a lot bigger. So it's not like a maximum number for the mirrors...it's just one point in time when she measures its power.
3. The evidence. So the sister has all this technology hooked up, all these computers, all this whatever, and in the end all it does is give the cops something to definitely pin the murder on the brother with. I'm really not sure what the sister thought she'd get with the technology...she knows the mirror can manipulate technology, and she also knows that a lot of the hallucinations wouldn't get caught on video, so I'm not sure how any of that could be used to prove the mirror is haunted. The temperature readings are a little more legit, but I'm not sure how she was formally tracking them to be used as evidence.
What I really wish, in movies like this, is that people would remember they have pen and paper. Sister could've written some crap down on paper (here's my theory, here's my evidence, I'm gonna go prove it) so people had a better idea of what she went into. Or written stuff down while she was in the house (so plants are dying, omgz we moved the cameras, ahhh scary ghosts) to leave behind more traces than just video. Yes the mirror would likely mess with her writing too, like it did with Dad's earlier on, but crazy cryptic writing tied with weird motions in a video paint a clearer picture of brother's innocence than video alone.