The Main Question: Can anyone suggest why I'm unable to install any new plugins? A (probably too long) explanation follows, and thank you in advance:
I've been helped kindly on the issue that appeared to have begun this long road of discovery in this thread:
But things have evolved in a new direction.
The problem began with the visual editor going wonky. It actually started with what I thought was a common WordPress issue--the editor reformatting html code. I was having the problem in several sites I work on, and "thought" what was going on in my own website was an extension of the same issue. Apparently not.
Some time ago, a fact which only returned to memory after things escalated on my website, I began having trouble with plugins. I recall that it started with NextGen Gallery, which I subsequently removed from my site (though the gallery files are still there). I then had issues with things going awry when Google Translate was active, so I deactivated and removed that (though the gt_cache directory, which is HUGE and delivers multiple error messages when I try to download the files into a local backup via ftp--I discovered that only today, was apparently not deleted when the plugin was uninstalled. SIDE QUESTION: CAN I DELETE THIS DIRECTORY AND COULD THE ORPHANED FILES IN THERE BE PART OF THE LARGER PROBLEM?)
That is all distant background, though. More recently, in addition to my html/visual editor issues(*), I've had "cascading plugin trouble". I've removed them as they had issues, thinking each time that it was the plugins themselves. Most recently, Contact Form 7, which I'd used without issue for years, started having problems, and I wound up trying to uninstall and then reinstall that.
That was when I discovered that I am unable to install ANY new plugins at all. They usually appear to install and activate just fine, but when I go to the settings pages to configure them, there is no structure. One plugin programmer said that WordPress was not loading the style sheets. I'd get text on the page, scattered at random, which are apparently the various choices and options, but no way to select anything, no buttons, no format or framework for the settings. The plugins themselves, on those which are just "install and go" types, simply don't work.
In addition, my usual backup plugin, WP Backup, was no longer, well, backing up. Its settings say that it's configured to email me a weekly backup, but it hasn't been sent in weeks. When I try to do a manual backup, I either get error messages, or the file that is download is just an "edit.php.html" file with a string of error messages about "no such file or directory" and "cannot modify header information".
I have tried numerous things, suggested by various people, to get to the bottom of the issue.
Today, I did a manual back up of WordPress, the complete structure, and then followed instruction found in the Codex for doing a manual reinstall. As instructed, I kept only my wp_content directory and subfolders (plugins, themes, etc). Before doing this I disabled all plugins and bumped the site back to the 2011 default theme (which I've changed since to 2010 because I really need to have my sidebars showing on pages and individual posts).
After doing this, clearing my browser caches, and rebooting the computer, it APPEARS that the visual/html editor problem is fixed.
However, I am still unable to install any plugins (the permissions for the plugins folder is set to 755, which I assume was done on install). The symptoms remain the same as mentioned above. Settings pages don't load and, though the plugins appear to install, they do not work.
My OLD plugins, for the most part, at least those that I had pared back to which were still working after these issues began to cascade, do seem to work. (At least those I've reactivated so far, though I confess to being leery of reactivating too many at once). I currently have Akismet, my Mailchimp sign up form, & Twitter Tools working. The two I'd attempted to install today without success were Easy Contact Forms and Ultimate-TinyMCE.
Thanks again for any help that you might be able to offer.
(* The editor issue that now appears to be resolved was that all pages and posts, no matter how I wrote them or saved them, would load in the html editor. I could switch to the visual editor and use it "normally", but as soon as I saved or published the post/page, it would switch immediately back to html. A relatively minor inconvenience, but it seemed that it was just the tip of a larger iceberg.)