Oh boy, the dreaded "aperture pin under the actuator" problem!
OK, if this is the problem you will have to do the following:
1. Put in the darkslide of the magazine and remove it.
2. Cock the shutter. Make sure the lens is almost fully mounted but not locked on the mount. IMPORTANT.
3. Place the shutter speed on "B".
4. Now, you will have to do the following steps WHILE depressing and HOLDING the shutter release button (don't let up!).
5. Look into the aperture from the rear of the camera and try to locate the aperture stop-down lever that sticks out from the rear of the lens.
6. It is probably hung-up under the curved aperture apron that is supposed to press the lever during exposure.
7. Take something long like a chopstick and press the lever down (toward the front of the lens) to clear the aperture apron and then rotate the lens to remove it.
It helps to have 3 hands or a plan on how to use just 2 to make this work.
IF you release the shutter while your fingers or a chopstick is in the focal plane shutter path, you might just ruin the shutter.
DO AT YOUR OWN RISK; I take no responsibility for your actions!
See if this allows you to remove the lens.