FIXED! It works!
Following the procedure in reply #34 worked well for me (uploading new firmware to the 8u2 via dfu-programmer). My IDE is responsive, minicom and screen have no trouble opening /dev/ttyACM0, and I can use the serial monitor and upload code without issue, even while streaming serial data constantly at 115200 bps.
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Thank you stimmer for sharing, and a big thank you to Dean Camera at the LUFA project and David Mellis @ Arduino / MIT, and everyone else who's worked on this.