Second, comparing the two NoteColumns in your example I notice a
different positioning of the accent in y-axis direction, but no
remarkable space changes between it and the cautionary accidental. Do
I miss something?
You are right: given that the cautionary accident introduced extra unwanted space, I assumed therefore that this extra-vertical space was made by some invisible horizontal extent of the cautionary accidental, but the effective unwanted space is between the accent and the notehead.
Though, the suggested override for vertical-skylines doesn't solve it
completely, see the ouput of:
{
%\override Staff.AccidentalPlacement.right-padding = 0.25 %% default 0.15
\override AccidentalCautionary.vertical-skylines =
#grob::unpure-vertical-skylines-from-stencil
<>-\tweak extra-offset #'(-1 . 8.5) _\markup \draw-line #'(10 . 0)
c'''!-> s c'''?->
}
Thank you very much!
Best,
P