> Take a look at https://www.youtube.com/watch?v=voKxhxxUmok for
> that feature, please. Usually I use Rolling feature.

Rolling edits is exactly what I want! Thanks :) So that solves part two.

Part 1 - it seems like "Drag and drop editing mode" might be a better fit than "Mixers" for your use case. I do not see any major programming features being implemented in the near future until some really motivated programmer retires ! (Thanks again, Igor, for Part 2 solution).