So much of what you have written here does NOT happen on my PC/Laptop.
Using "Convert Video Clip" sends the clip to the Video Converter where full editing features are offered. The Clip is converted and added to the File List.
If I use "Add Selected Files" to add Unconverted Video (instead of Dragging) - I am offered the OPTIMISATION window and if I accept the Optimisation Icon appears in my Taskbar. I can then access the Video Converter and Stop the Optimisation/Conversion in order to TRIM the Video(s). However, in the case of the "Add Selected Files", this will not alter the Slide Duration because the Slide Duration is SET when the Image/Video is added to the Slide List. The Converted File can be add to the Slide List and ITS Duration will then set the Slide Duration. As others have said, I assume that you would not want to alter Slide Duration in a slide containing multiple Objects if a Video Duration is changed by Trimming.
NEVER happened to me.
"Convert Video Clip" does what it says - it converts an Unconverted Video Clip and ALWAYS offers the Trim Facility. If the Conversion started immediately then I supect that you might have started a conversion and Paused it or similar - invoking the "Convert" command re-started conversion.
"Add Selected Files" can apply to both Converted and unconverted files and the appropriate action is ALWAYS consistent.
So much of what you have said should REALLY be in Suggestions for Next Version but you might even get some objections there.
I have Unticked the Notifications for this thread for the sake of my sanity.