I think you may be overlooking something here. If the crank will move a quarter turn either direction, then the motor is not seized. Something else is jamming up the works.
Did you try to turn the motor when the clutch was fully disassembled? This will tell you if you have crank problems or something else. In other words, disconnect the motor from the transmission and see which area is the problem one.
I have had similar issues with motors before, and the problem was usually something to do with the clutch being incorrectly mounted, or gear selection shaft mechanism being misalined and binding up behind the clutch basket allowing only limited movement before locking up. Sound familiar?
There has obviously been a fair bit of damage caused behind that clutch basket. To snap a hardened steel gear in half like that takes a LOT of force. I would be carefully inspecting other components in that area for damage, particularly being bent or not seated presicely in position.