Greetings Jill,
I understand all that you have mentioned and I have done those steps. In this show, I have already put in all the still images that I want to use. Then, I am inserting the short video clips from the long video file. Yes, I can drop in the long video file, then roughly Trim the Beginning. Since I already have my still inserted in the Timeline (and the next slide is way, way far to the right), I have to drop down a dummy image where I want the long video clip to end, roughly. Ok. Then go back to the Slide mode and remove the dummy image. Then to trim the beginning and ends of this short clip more precisely, I bring it into the Adjust Time Range function. But even though the new video clip is only 20 seconds, the entire remaining long video is still there and I have to drag the far right triangle roughly to where I want have the final trim established and use the Start Time/End Time box to do the minor adjustment.
If one does work with long video clips, I understand the lack of understanding with my work method. I prefer to do an actual trim and save the clip as its own file and it show up in the File List. I don't think my suggestions would affect anyone in a negative way, but actually make it more intuitive and easier. The Trim function can be made to be more logical and useful for trimming long and short clips.
And, using the Convert/Trim process, as I prefer, allows you to use different Convert Presets for each trim if you wish.
Gary