there's no way forward for a challenger in the primaries.
A challenger* is inevitable. The Democratic Party is far too diverse for a candidate to satisfy everybody in the early contests. As the presumed nominee, Clinton will not be able to take a lot of stances that satisfy a lot of the more activist types, which creates space for a challenger, and whoever this challenger is will get coverage by the media.
More than likely this challenger will be some loser congressman or something that has no chance of beating her, but I would not be surprised if Clinton was held to around 60-70% in Iowa.
*and by challenger I mean a non-negligible challenger, not a Randall Terry type.