What exactly is a pre-trial diversion
It can vary from state to state, but generally it's a way to agree to certain terms and conditions (think of it as prospective probation). If you meet the terms and conditions, no trial, no conviction, no record.
Pre-trial diversions are generally agreed to between the prosecutor and the accused under two conditions - first, when neither side is completely comfortable that they can win outright, second, when the accused has a clean history and good prospects for staying out of trouble if given the chance. There are also, in some states, laws that require pre-trial diversion be offered for certain crimes. However...
I suspect the "not sure of a win" scenario in this case - pre-trial diversion is generally something that is not offered for crimes of violence unless the prosecutor is not sure he can secure a win.
Again, that's speaking generally - I don't know the specifics for Nebraska.