What I want to do is to delete one quarter note at a certain place in the score. I thought it should be possible to select the quarter note across all staves, turn on insert, and press delete. But I don’t succeed. The staves that actually have a quarter note to select will do, but the ones that do not or have a rest will not be affected. How can I achieve what I want?
While this doesn’t address your problem exactly, just for your information: If you use the Shift+B popover to delete entire bars (with -1, enter), this will actually affect all voices across all staves (it uses insert mode internally).