A similar thing happened to us at our inner city Mosque of all places a few years ago. We were hosting an Iftar there and a friend of mine not used to this Mosque came. She left her wallet in the front Musallah area (everyone used to leave their purses all around in different corners at this time), where there were a number of ppl around and kids playing and then went into the back to get food, then we came back and ate and prayed etc and at the end of the whole evening she realized her wallet was missing! Hate to say this but during most of the time there was one brand new convert girl always near the front looking suspicious. Hate to say this as well but I was always a little disbelieving of this girl too over the widely inconsistent things she told us about herself and how she converted, etc, but I was always nice and helpful to her just in case lol!
Anyway, the next day someone took the credit card and bought a whole bunch of things (girl things like boots and expensive stuff) and then tried to use it at an ATM where the suspect got locked out. The convert girl never came back to the Mosque, don't know if she's still Muslim, but I saw her on the street once looking totally different and she avoided me like the plague. So in the end I think it's probably wrong to suspect her, it could be anyone, and I think we should just blame ourselves. Now we leave our valuables locked up in the car which is kind of sad, but has to be done.
Anyway for your situation Allahu alam what happened to the money. To be honest anything could have happened to it. I've been so sure at times that I lost something in a certain place ie like a ring at a wedding, I even went back to the hotel to look for it! But then I found it weeks later in my closet of all places!! So you could have misplaced it, someone else could have taken it. You could have moved it, you could have used it. It could have fallen somewhere.
About the girl you suspect I wouldn't be friends with her anymore, because you obviously suspect her and don't trust her, so there's just no point.
I think if someone steals from us it makes us so upset and angry and all those emotions you mentioned. But in the end we can accept it by realizing nothing really belongs to us anyway. Allah gives it to us and sometimes He takes it away to teach us something or gives the rizq to someone else. Perhaps someone at the Mosque felt jealous that my wealthier friend from a different mosque had all these things and she didn't and took it, well whatever reason ppl steal somehow they must justify it to themselves?? In the end it's the person who stole's misfortune and not ours.
There's a story about Rabia al-Adawiyyah and how a thief entered her house at night and she let him just take whatever he wanted, and when people asked her about it she said, Alhamdulillah he hadn't stolen her Iman. I think we should just think of it like that. Don't accuse anyone wrongly, don't do anything not Islamic. Don't even suspect with no proof. Just accept the loss, learn from previous mistakes and move on and try to improve yourself in the sight of Allah, inshaAllah.