Do estimates have to be included in the forecast or can the opportunity drive the forecast only? Larger orgs can have 1 opportunity in the forecast and the estimates are not part of the forecast. In this scenario, the opportunity is created for the lowest estimate.
If you want to approach this from a non-system method and bring in forecasting rules to guide sales, then a salesperson submits justifications for why multiple estimates are necessary.
Typically in scenario 2 the rep only gets 1 estimate and complains. The underlying sales behavior is the issue. The sales rep needs to press the prospect WHY this is required. When a prospect asks me these questions I make a little simple model in a slide deck.