Is it possible to add multiple slow motion ranges in a video instead of one single range?
Only one slow motion interval can be set at a time. If you want multiple slow motion intervals, you need to export them in batches and edit them with other app.
No. Changing frame rate mid-video would need reconfiguring of sensor on the fly, and it won’t be a seamless experience. It may even need recompiling of video if the frame rate is changed from a lower to higher one. It may be possible with much effort, though.