The "patience thing" seems to be working. After four days, I finally got one of the over riders detached from the bumper. Consumed one can of PB Blaster, some WD40, some Lucas penetrating oil spray recommended by a friend who uses it on his old Lotus and Corvette, a brass wire detail brush for cleaning bolt threads, and bought a set of offset box wrenches at the "disposable" tool store Harbor Freight for better leverage,
The second over rider is soaking and I'll try again tomorrow morning around 6:30 AM while the temperature is still under 100º. It's not a hobby, it's an adventure! 😎