Topic update;
For anyone out there is still in deep worries and having sleepness nights about this issue it turned out to be a linkage issue afterall as well in my case.

The horizontal rod on the transmission going from the KD-lever to the bellcrank was too long.
This caused the rod to contact the bellcrank shaft at full forward/relaxed position, and prevent the KD-valve in the transmission from totally releasing.

I put an extra bend in the rod to shorten it which freed up the linkage quite a bit already.
It still needs to be slightly shorter as the bellcrank's upper leg is almost in it's upright position, almost causing to overcenter the vertical rod going up to the engine bellcrank.