Thank you for responding!
It seems that v18.30.00 has been removed and only v18.21.00 is available for the 7620 at the moment.
Although the Win10x64 driver-only installer package didn't work (as I detailed above), I was able to install v18.30.00 using the Win8.1x64 driver-only package.
If possible, please let me know if there are issues with the new driver that would make rolling back advisable, or if it was removed only due to installer problems. I'm sure I'm not the only one that downloaded and installed v18.30.00 while it was available.
I added a clear statement of the issue to the original post. If you still need info, could you ask specific questions? I don't know what else to add.