The bar that keeps the seat in place is held in a slit and is spring loaded so if it catches, it won't open all the way.
I'd turn the key then try to move the seat down and forward. One thing you might try is some WD-40 with a straw on the tip. Wedge the straw down in there and spray it around.
Maybe spray some into the key hole.
The seat tends to move a good bit, moving it around once the key is turned is about all you can do.
After that, maybe cut a hole in the bottom because you can always epoxy it back up.