It is true that managing duration in a project is one of the hardest tasks for the PM to handle ideally. Even though all task durations were arranged and planned precisely, some out-of-control issues could appear at one point and require a timeline rearrangement. Therefore I agree with the initial statement that having every manager from each discipline to meet and discuss time estimation for each phase is an essential step in the planning process. On the other hand, I think everyone involved should optimize the simultaneous tasks that can be achieved from each department and each phase. This will enforce a more rigid framework for the project. Also providing strong communication channels in order to get periodic updates on the work process, this would allow working on different tasks while modifying the failed one.
However, I think finishing early should not be a concern but if it happens I think more time should be devoted to the product validation.