The parts on that motor should slide together easily and require no force at all...none. Something is wrong if the parts do not fit and rotate freely by hand. Leaving the star and the mating rotation part (internal gear) out but with the shaft installed, does the shaft turn free? When apart, did you sand/polish mating surfaces lightly with fine emery cloth (and clean completely)? Go one part at a time, putting it together and find whatever is binding.
Personally, when I put a pump or motor back together, I coat these surfaces with white lithium grease, but not without them rotating freely first. Others may just use oil.
Obviously you proved there is pressure, so the valve and pump are working. Seems your focus is narrowed to the motor.
On the drum lever, have a friend rotate the winch drum as you put the handle into the engaged position. Maybe a 1/3 turn of the winch drum will get it to align, where it will engage (this is not a clutch, it is a gear that must mate). If the motor was turning... this engagement would make a difference, but you said you had pulled the motor and it was not rotating.