G-flex into the crack between the sole and the shoe body (if my eyes perceive the problem correctly), I would probably put a couple of layers of Kevlar down the side of the shoe body and down under the plate bearing the blade towers. Kevlar doesn't stretch much at all and is very strong in tension.
You would have to scuff the black surface off the leather, and it's possible the blade towers will leave too little space for the Kevlar. If there is enough space, that surface would have to be cleaned and roughened.
The reason I suggest the Kevlar is that I suspect it will not be possible to clean the inside of the opening crack sufficiently for G-flex to get a grip.
Other glues? I have some black Shoe Goo which is much better than they used to supply. It is thin enough to work inside of that crack.
Another stupid idea. Maybe you could aim a drill down the side of the heel area so you could drill through both sides of the crack. Then you could select a machine screw with a low, round head, and pull things tight with a washer and machine screw on the bottom side. I'd do that in addition to the glue.
EZ-Dock modular docks
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