love your expression "triple-pronged tiara" Sonds much better than leaf tension assembly.
Does yours have this button on the base? If so if you push it down it may release a latch from the treadle.It will allow you to tilt teh machine back on the hinges. That will let you see if you have a shuttle, give you better access to your stuck slide,and give you access to the two set screws that should be holding the head to the hinges.
hmmm attachment is acting up. I will try the pic later.