For example, if a user loads a video, watches 10 seconds of it, and then scrubs it all the way to the end, is the bandwidth charge/usage calculated as if they had played the video all the way through?
When viewing a video the video is downloaded in segments. When you scrub a video to the end, you don't necessarily download all the segments. The bandwidth usage and charge is only for the segments that were downloaded.
The amount of HLS segments served to the end-user are considered bandwidth usage. Scrubbing advances the video to later segments and adds the remaining segments to the bandwidth usage calculation. If segments 1, 2 and 3 (each 10 seconds long) were downloaded and the user then the scrubbed to segment 50, segments starting at 50 and onward would be considered as part of the bandwidth usage and cost. The total usage would be for segments 1,2,3 and 50+.