Sorry about the long wait, had severe storms last night and finally just got power back.
Well, at this point I am at a complete loss. I know how to probably get you past this screen to complete the install, but I now I don't know if it will activate. Before running this procedure, I'd double check and make sure you have extracted the EXACT 25 digit product key extracted from the Venue 8 Pro's BIOS using something like Produkey and that you haven't written it down incorrectly. (I have my doubts about the 2nd product key Dell gave you since the version of Win8.1 on the flash drive wasn't even the right one for your device - if Dell support couldn't even get that right, who knows what that new Windows product key Dell ended up giving you actually is for...).
If you still want to try it to see if it works, you can do the following:
1) Cancel the install and shut down.
2) Plug the USB drive you created back in to your computer.
3) Create a text file in notepad (it has to be a text file) and add the following information to it (you can copy and paste):
[EditionID]
[Channel]
OEM
[VL]
0
There should be a blank line under EditionID. Save the file to your desktop as ei.txt, then right-click on ei.txt and rename it to ei.cfg (you'll get a warning about doing this but don't worry about it).
Copy and paste the ei.cfg file to the "Sources" folder on the flash drive. Then, try to boot the tablet using the flash drive.
This should change the install so you can either skip entering the license code, or it may not even ask you for it until after the install is complete.
But again, this still raises the possibility that it still won't activate after install.
Its your call as to whether to try it - that other suggestion I made might be worth further consideration (though, we don't even know if that would work either).
I'm really starting to get... annoyed ... with Dell about now.