Well, 24hrs later and it's back to square-one. So no amount over-tightening or fiddling with torx screws fill fix this problem. A light wipe over the sensor area brings it back just the one time; a long, heavy press on the right-hand corner seems to fix it for hours-on-end. I can only assume all the pressure exerted from the back with the cover off had the same effect as pressure on the front.
When I clean the entire screen using relative pressure, the double-tap also fails - so the screen may lifting a fraction during this process... of course once the double-tap fails the blacked-out call screen emerges also... :angry: