are you recording in FMR?
If you're recording in 'automatic' Captivate basically takes a screenshot of your screen/application and superimposes the mouse movement. When something changes or you click, it takes another screenshot. Once those screenshots are compiled you can move and manipulate the screens and mouse movements as needed, including removing entire slides.
The way I understand the Demo, Assessment, Training options is those determine what type of captions and and clickboxes are available in your finished product. Those options shouldn't disable you from manipulating the length of slides, only how the learner interacts with the slide.
If you're using FMR you're stuck with the length of the slide as it's recording 'real time' and you don't have options to manipulate the timeline (well you can make it longer but it doesn't impact the FMR movements).
If using FMR you can use the Split and Trim options to chop out unwanted sections of the recording.
This accordion is only present on FMR slides. If you've recorded the simulation in standard demo mode, you can adjust the duration of non-FMR slides just by dragging around the objects on the timeline and shortening or lengthening the slide timing. If you haven't used slide audio but chose to add audio over a number of slides, this may mean you'll need to readjust the timing of slides to match the audio again.
I stand corrected, I'd never seen that option. Looks like I learned something as well.