1.) the British cruisers disabled Admiral Graf Spee. E.g. here oil purification plant and desalination plant were destroyed (both drastically reducing her range) and her bow damaged. She was not considered to be able to sail home (plus she was low on ammunition), therefore she was scuttled.
2.) I do not think the the armour of Admiral Graf Spee could defeat the 8" shells. She was only a cruiser and had a 50-80 mm belt (2-3.1" compared to 4-6" on Baltimore) and 18-40 mm armoured deck (0.7-1.6" compared to 2.5" on Baltimore)
3.) River Plate was fought with optical fire control. In 1944 the USN radars were much more advanced than the German, both the search radars and the fire control radars. Already in 1942 were British cruisers clearly superior to German cruisers regarding fire control, e.g. at the Battle of the Barents Sea. In 1944 Admiral Scheer would have needed luck so that the way superior fire control and radars did not matter. Likely she would have been surprised (as Admiral Hipper and Scharnhorst) and heavily hit before she could even react.