TBone! 'me dear chap. You have given such good info to those of us in need. I have one or two suggestions.
See if the keyboard has an XT-AT switch stuffed away somewhere - if so change it to XT and re-boot.
Also, see if you have accidentally bent a pin while connecting, it happens as you know.
If neither works, it may be that you have duff board and it might be worth a trip back to whence it came.
Oh! Have a look in Device Manager to check that no question mark or exclamation mark is there.
Thats the best I can offer, not as succinct and good as your advice, but I do 'me best.
Hope it works anyway.
Take care.