"When you run the Windows setup exe for it, are there any customization options that allow you not install Opera?"
No. It just does an automatic repair/update The only way I even know it was working on the correct directory was by checking the dates on the files.
As for Easy Diffusion, it's not a simple matter of telling it not to install something. It does a full update of all files each time it's run, and hidden somewhere in all that is the Opera update. But I thin I have narrowed the problem down to just a memory issue. The problem with it not starting seems to have resolved itself and after several reboots, it seem to be working normally again. I did update my video drivers a few days ago, so that might have something to do with it. The lost passwords problem seems to be gone now too.
Avast and Malwarebytes both came up empty.
All that's left is the loss of drag & drop functionality, which seems to be memory related. I lost that again this morning, and that was after Opera ran out of graphics memory for pages, which I loaded it up with to see if that would break something, which it did. Stable Diffusion is also a graphics memory hog, so running low on that seems to be the cause of the problems.
Thanks for the info on importing the passwords. That's going right into a text file for future reference!