Rather than creating an InstantMovie based on the clips on your timeline (by selecting the standard InstantMovie tool), select the clips you want to use in an InstantMovie in your Project Assets and then right-click and select the InstantMovie option.
The program should keep creating InstantMovies, one after another on your timeline.
Please explore the following solution....
Create your Instant Movie as you have on the Timeline Video Track 1 (Instant Movie Tab at bottom of Expert workspace), however, in the Setup, remove the check mark next to AutoEdit.
In the screenshot below, the check mark has not yet been removed next to Auto Edit.
That should resolve your issue as I understood it.
Looking forward to your results as your schedule permits.