The J-K-L buttons for controlling playback of video is something that comes from Avid MediaComposer. It has been copied by Apple in FinalCut Pro and Adobe Premiere Pro. Apple has got it right, Adobe only partial. I suggest that it is changed in Kdenlive to work the way it should work.
Press J once - play backwards
2 x J - backwards 2 x speed
3 x J - backwards 3 x speed
4 x J - backwards 5 x speed
5 x J - backwards 8 x speed
Press K once or as many times you want - pause. If the video is not playing, it remains in pause. It should NOT be a play/pause toggle.
Press L once - play forward
2,3,4,5 is similar to J, only forward.
Hold K pressed and press J at the same time - play backwards at 25% speed (including the sound).
Hold K pressed and press L at the same time - play forwards at 25% speed (including the sound).
There is no need in having play/pause as a toggle, as this is something most people expect to find on the spacebar. Also, pressing L will start playing, so no need to have that on K.