After all this testing, did you ever see a log or anything else that told you exactly why the system is being blocked? I have not seen that message on any of my systems, although I do have one which has not yet been offered the update and no message as to not being capable.
When you did the second upgrade, did you start with a clean drive or could something been left on the system from the previous install?
The online upgrade does download packages for both the .382 and .521 versions, so being offline may not be the best way to go. It appears the in-place upgrade does not download the .521 update package, but I get that from your and other's comments.
One change might be, that the problems with the .521 update, Microsoft may have taken some steps to wait until that situation has been worked out. If I haven't seen the upgrade offered to my second machine by next Tuesday, I will try the .iso upgrade path.