If the witnesses don't show up the case gets dropped in America almost always.
Not really.
Its on the prosecution to determine if they want to prosecute with or without a witness. If they feel the evidence is still strong enough they can certainly move forward with this case. The defense can always make motion for dismissal but that doesnt mean the case is going to be dropped.
Also a witness can be subpoenaed to testify and if they fail to do so the judge can slapp them with a contempt of court charge. The witness does have a right under the fifth ammendment to plead the 5th. Basically saying hey im not answering any questions because my answers can implicate me theres probably other reasons why a witness can invoke the 5th but none come to mind.