OK, I could finally put everything together and get the pinch to close working in a Wayland session, even for KDE windows (using dotool) 
Unfortunately, the KDE default gestures (especially the redundant 3-finger swipes left and right) are not overridden, they are clashing with custom gestures.
About this:
the explanation is certainly that you are using a single worspace, hence the inactive swipes left and right and limited usefulness of 4-finger swipes up and down.
Correct?