As you havent answered the question of can you deglaze a window how can i help properly ?

Ok lets assume you can deglaze the window then drill through to the left or right of where the spindle goes into the locking strip. By drilling through a small way you should be able to see the slider and move it back with a screwdriver.You can then open the window and remove the mech and replace.

Another way is to use a wide flat edge to lever a gap open and knock the slider back this way.I have tools i have made just for this job so not always as easy for a diyer.