Hi Ian,
Hm, I'm able to just click and keep holding my mouse as I drag over the others. I use a real mouse, not a touch pad - although doubt that would make a difference?
As far as the design, I'm not a UX designer nor on our product development team, so I wouldn't want to weigh in on the why regarding design. I will share the information here in the form of a feature request with our Product team about how it's difficulty for you and others to work with this set up.
As far as the playback lag, that is something we've shared with our team in the form of a bug report and although there have been incremental improvements with recent updates of Storyline it's still an outstanding issue that they continue to investigate. I'll be sure this thread is included in that report so that I can update you here once there is additional information to share from our team.