Where it comes to rotoscoping there are many approaches.
Firstly, you may want to describe your difficulty with video import as you should be able to do that.
I can actually import video more easily than image sequences.
An option that many don't consider is that of using a program/utility to make a program see-through.
With such a program we can draw in OpenToonz directly over the top of programs running underneath... even video.
There are programs that have native ability to go transparent such as 'Made with Mischief', although that isn't an animation program.
But all of this is outside of scope.
I think we just need to resolve the issues you are having because rotoscoping is accomplished all the time in OpenToonz.
Have you actually tried to rotoscope in OpenToonz or is what you are referring to just what you've gleaned from the documentation?