Firstly I have zero knowledge on this specific subject, but have a pretty extensive mechanical background.
Maybe I am missing something here. The diagram shows 8 clutches and 7 plates between them. Your pics show only 6 clutches and 5 plates. Are there different thicknesses for different applications? I.E. Dirt Bike vs. Kart that would have more or fewer plates?
Found this video on rebuilding one:
Brought up a point about the plates having a flat side and a tapered side. Could that cause a clearance issue in the overall thickness of the stack not allowing it to fully release? Also talked about the springs having a certain spec. Could they be incorrect in either length or resistance? This could explain why it is so hard to actuate the clutch lever by hand. Last thought is that one or more of the clutches or plates is binding with its contact surface and not allowing the stack to separate.
Just some thoughts. Let us know how it goes.