5. As a final test, I run "Duplicate" in Finder, and it spits out an I/O error (which I confirm with a second run, because I've gotten false negatives).
Are the results of step 4 inconsistent with those of steps 2 & 3, or am I misunderstanding what "Verify" and "Checksum" actually do?
What exactly are you having Finder duplicate? To where? Is there enough space?
Finder won't be able to duplicate a DVD (no place to put the copy) nor files on a DVD (destination would be the same DVD, which isn't writable). I couldn't find a way to make Finder even offer "Duplicate" as an option when I select either a DVD or a file on a DVD. (I assume you're talking about the File->Duplicate menu command.)
Having Finder duplicate the disk image file the DVD was created from won't tell you anything about the DVD itself.
If the DVD is using a filesystem that does not natively support resource forks or other non-datafork metadata, files will be stored in AppleDouble format, with the data fork for a file named X stored on the DVD as a file named X, and the rest of the metadata stored in an invisible companion file named ._X. When the file is copied back to an HFS+ disk, the dot-underscore file is supposed to be merged back with the original file, but sometimes that doesn't happen. The problem seems to arise when the destination folder already contains a ._X file and/or an already existing file X. Finder sees metadata for X coming from multiple sources, gets confused, and reports an I/O error.
Try dragging files/folders from the DVD into
a brand new empty folder, one that you're sure doesn't contain any invisible dot-underscore files, nor any duplicates of the files you're copying.
If that helps, look at dot_clean(1) for additional help.
There is also the possibility that the I/O errors are legitimate. Finder may be encountering honest-to-goodness errors on whatever drive it's trying to write to. (The fact that Disk Utility can compute a checksum tells us the DVD can be read without error.)