Scrolling panel with layers appearing over image at certain points along the scrolling

Hi all, I would like to use scrolling panels (but I am open to other solutions that do deliver the same effect, if you know any) to teach to the learners how to fill forms that are long, multiple pages (Pdf). I would like them to be able to scroll the pdf in a scrolling panel (this I have done many times and I know how to compose the long documents inside the panel). What I would like to do, is to have some dynamic (meaning, not static overlays on the Pdf images of the document) boxes (like...red rectangles that show different areas in the document) that appear as the learner scroll the panel. Like a red empty rectangle over some fields in the form, may be some arrows and highlights also. Ideally, I would also time it with synchronized voiceover chunks (but I am not sure if the voiceover can also stop if one scroll the panel back - possibly this is impossible). Maybe an alternative could be to build this as a mp4 with chaptering on the timeline, and insert it into Storyline, but as this is time consuming, I was wondering if there was a way (JScript?) to do it with the built in scrolling panels and some triggers that invoke layers. It would be (if possible) more elegant to be able to scroll a document in a panel and have overlayers that appear every time the learner scroll up (but also down), rather than asking them to move to a specific 'chapter' in the timeline of a video. The latter would be my backstop solution. Has anyone ever solved this technical set-up in Storyline360? Thanks in advance for any idea.