Well... I had the very same problem here as op described in his first post.
My case.
No way to install Opera 76.0.4017.123, but I found two workarounds.
Situation:
Fresh and clean install of Win7 64bit + updates + ASUS motherboard drivers. Nothing more than these.
No way to install Opera, either using standalone installer or downloadable version. Actually download version starts downloading installer files and than get stucked at some point (I guess at the beginning of installation process).
Solutions I tested in order of appearance 
- Switched to Windows 7 Safe Mode, installation went without any problems. I restored Windows 7 from image file in order to further investigate
- Disabling Windows Firewall and Windows Defender, restart, than installation went without any issues using download version of Opera. I did not further investigate which one has caused installation issue, Firewall or Defender.
After going back to normal mode (Windows 7 Firewall and Defender enabled) the update button is grayed out as described by op in his first post...
So this is actually request to Opera development team to check what's going on, even though the Win7 is not supported (?) any longer. 
I hope my post will help someone.