Yes. Circumstantial evidence can be enough to convict someone. The question to the jury will be did the State prove its case beyond a reasonable doubt? It will come down to what is the actual evidence and can there be interpretations of the evidence other than that the accused committed a crime. The defense attorney will likely question the State's witnesses about other possible interpretations of the evidence. What those questions will be will depend on the exact nature of the evidence.
Yes. A person can be convicted on circumstantial evidence alone. AS you a pointed out in your question, it is a lot harder to get over the hurt a lot of beyond a reasonable doubt without direct evidence.
Yes. One can easily imagine a set of circumstances in which there is no direct evidence of a theft but evidence of access to the money, opportunity to take it and signs of new found wealth that point to a particular person. It may be that the state cannot convince a jury of the accused's guilt beyond a reasonable doubt but I would guess most attorneys on this site can tell at least one story in which a defendant was convicted solely on circumstantial evidence.
If you havent already, please speak to an attorney, or have the accused do so, as soon as possible.