In yet another configuration from the Gigabyte Llano motherboard on the Linux 3.1 development kernel there was visible memory corruption, but this time actively changing.
Being time to change motherboards, next up it was the Platinum A75 motherboard that was kindly sent over by Sapphire Technology. While the Fusion motherboard previously had mode-setting issues on the open-source driver stack, with this very latest code, and when connecting it to the Dell P2210H display, the system was finally working! Kernel mode-setting was working right and the Gallium3D driver was functioning. The software stack had remained the same, but the motherboards were just swapped out.
Once those issues were worked out, no other show-stopping problems were encountered on the open-source Radeon Linux driver stack (Mesa / DDX / kernel) for the A8-3850 APU on the Sapphire Platinum A75 motherboard.