Thanks for reposting with pictures, so much easier to see the problem. This is what I would do...... you need a styrofoam head form, most collectors have them so I am going to assume you do. Put a plastic bag over it so your liner won't stick to it. Next take a couple of sheets of paper towel, put them together and fold until you have a double thickness strip of Ptowel about 2 inches wide. Wet this with water, pull your liner up out of the shell. Tip the helmet up on an angle, if you have a rectangular container (to hold it) put it in that tipped up and facing you. Next, place the wet Ptowel strip along the back rim of the shell where your problem is but not on the liner. Leave this in place for approx an hour. What we are doing is wetting that particular area which has collapsed and not the entire helmet. The water from the paper towel will soak into the leather making it soft. Once this has happened put your helmet on the head form, making sure that that rear area has been pushed out to where it should be to match the circumference of the rest of the shell. Allow to dry for 24 hrs. This is called "blocking" If you had a hat stretcher, you could leave the liner pulled out, avoiding any moisture contact. Then, put the stretcher into the shell and push that area back into place. I use antique ones but there are modern ones available on Ebay. This is an easy fix, you should have no problems.