"Reasonable belief" has no role in a preliminary exam. You may also be thinking of "reasonable doubt," the standard that controls the jury's inquiry. At a preliminary exam, the prosecution only needs to show evidence of each offense, and not guilt beyond a reasonable doubt.

You can't skip to sentencing until you have a conviction. You can only have a conviction if a defendant pleads guilty or is found guilty after a trial, and a trial invokes a variety of legal rights and procedures that the media often dismiss as "technicalities" (including the right to a jury). Sentencing someone after a preliminary exam would be like declaring the winning pitcher after the first innning. In roughly 5% of criminal cases, the jury ultimately acquits a defendant. In a larger percentage, they convict defnedants on lesser charges or they accept some charges and reject others. But this is done only after a full trial, in which witness credibility can be fully explored, and where the defense has had time to prepare (in most states, the preliminary exam must be conducted within just a few weeks of arrest).