The OS only becomes aware of the license when you enter it at installation time. A valid COA = a valid install.
The ISO is unaware of the license, you cannot use the OEM key, but the key on the COA sticker works fine and is authentic. Win7 will install using the key on the sticker.
The question is Does MS allow people to download the ISO with the COA on an OEM machine?. I think MS does NOT allow the download, but things change.
Uninstalling the two Win7 KB for GWX will remove the icon from the taskbar, but they won't be on a Clean install system until WU offers them. You'll have to hide the updates to stop WU from offering them again.
If the GWX icon keeps getting installed, don't reserve your copy of Win10 and that should stop Win10 being offered in WU.
I just want to be sure that you tried all of the steps in this tutorial (some posts in the tut might also help) :
Windows 10 - Go Back to Previous Windows
Read through that tut and also review the Related tuts at the bottom of post# 1- there might be an answer in those.
How to Reinstall Windows 10 with this media
How to Upgrade to Windows 10
How to Upgrade Windows 10 Home to Windows 10 Pro
How to Delete Windows.old Folder in Windows 10
How to Refresh Windows 10
How to Reset Windows 10
How to Create or Download a Windows 8 and Windows 8.1 ISO
How to Start or Stop Receiving Insider Builds in Windows 10