The 2 witness prerequisite was given in the Mosaic Law to protect people from slander or false accusations by their enemies. Surely, you see the benefit of that prerequisite when someone accuses another of wrongdoing. Even in courts today, solid evidence must be presented in order to prosecute a wrongdoer. 2 witnesses would provide that solid evidence to convict someone in the congregation.
Unfortunately it is true that it is very frustrating for those victims that were abused and are the only ones accusing the abuser. In those cases you will agree that extreme caution must be taken to protect the victim, but also to make sure that the accusation is a valid one. You say:just turn him in to the authorities and let them decide. I think the organization has also looked into that argument and have told elders to not interfere with a person going to the authorities. So this is a positive step forward.
The elders can't even warn parents in the congregation that so-and-so may have a problem and it would be better not to let him near the childrenActually the elders do have the authority to warn parents of possible abusers if that person has been convicted (by a judicial committee in the past) of that sin. They do not make an announcement to the congregation, but they do give a talk in the Service Meeting a few weeks after the judicial committee about that sin without giving out names. The members of the congregation are then formally warned of the dangers of these predators. Each parent is responsible for the safety of their children. Elders have approached a parent and warned them (without giving out details) about not associating with a certain individual. It's up to that parent to follow the counsel or disregard.