Thanks to all for your response to my post. My intent was to manually upgrade to the newest version of Opera (without deleting my bookmarks). But regretfully, I can't reconstruct a detailed account of what I actually did. I'm now using Opera Stable for Windows 10 (64bit) Version 51.0.2830.55. The missing bookmarks, if recoverable, fine; but if not, I'll just start a new collection.
As far as I can tell, my Opera files include no problematic anomalies. But curiously, and for reasons unknown (to me), I have two application files in the same directory, i.e., D:\Program Files. One of these, "launcher.exe" (1226 KB) works normally (except for the bookmarks recently deleted by accident). But the other file, "opera.exe" (918 KB) takes me to: https://www.opera.com/client/welcome-old.