How do I replace a section of video in an iMovie

Put a black background into the main timeline and adjust its duration to match the length of the movie. Put your video clips in the upper timeline above the black background. Now they can be moved around or segments deleted without snapping together. Then you can add your replacement video in the gaps.

Another way is to use the main time line, but insert background clips as spacers to fill the gaps until you can add your replacement video segments.

You can uncouple the audio from the video above it by putting your cursor on the audio clip and, while pressing down, pull it straight down until the little attachment handle pulls free from the video clip.

-- Rich

Another way would be to put cutaways as the replacement clips above the main timeline.

Like you, I need to know how to edit, add to, rearrange content without messing up the audio track. Currently, the audio gets attached to a key visual and when I mess with visual content the audio gets out of synch. How can I 'detach' the audio from the visuals?