Ortlieb messenger bags aren't foolproof, or drybags for that matter though they are very good bags and they make even better panniers (the panniers, not the messenger bags though I have seen the messenger backpacks converted to panniers). The rolltop is good for expanding & retracting capacity too.

Cordura is nylon, the liner on a REload is PVC. Scotchguard may exacerbate your troubles as it could cause the water to bead more than it already does on the exterior, thereby channeling it more efficiently into the cleft between your back and the bag, where it collected well enough to saturate and deep enough to penetrate the seam where the shoulder is attached to the bag which experiences a great deal of shear force which could expand the stitches of the seam.

Get yourself a tube of Goop Marine or similar marine adhesive sealant. Apply liberally to the area where strap attachment meets bag, where they are pulled apart by the opposing forces of your shoulder strap and your load in the bag. Apply while pulling the two components apart form one another in the same directions you'd find it under load. Allow to set for a day. All done.