My first SQ-A body got into an altered state where the cable release would not fire the shutter, but sometimes the button on the front would. The condition rapidly worsened until the shutter wouldn't fire at all and I was in a situation with similarities to your own. I got the back off, but could not remove the lens. With the back off, looking in as seen here, using a small wooden stick, I lightly touched the stud-thingy in the curved slot seen at the upper left interior and the shutter fired. I was then able to wind the crank to bring the mirror down, cock the shutter and take the lens off. I have this vague recollection that there was an additional link sticking out that doesn't show here -- could be my aging memory, or could be because in this shot the body is not duplicating the original problem state.
Interesting to me, when I risked my 65 mm lens by putting it on for this picture
just now, I pressed the shutter button and heard a slight noise but the mirror did not flip up; when I touched it ever so lightly with a finger tip, it popped up. That may be a hint as to my problem, and you may find that condition when you get your back off.
Fun cameras! (Moreso when they work right.)
Good luck,
DaveT