Just wanted to let you know that I'm still here. I had a family emergency earlier in the week and finally got to revisit this yesterday.
There are two php.ini files on this server (2008 R2, IIS 7.0); one in C:\Windows and one in C:\Program Files (x86)\PHP\v5.3\php.ini. I made your recommended changes to BOTH because I'm still confused between "Configuration File (php.ini) Path," which points to C:\Windows, and "Loaded Configuration File," which points to C:\Program Files (x86)\PHP\v5.3\php.ini. As in, which is the "real" one?
Anyway, after making your recommended changes to BOTH php.ini files, I not only stopped and restarted the website through Internet Services Manager, I also stopped and restarted the World Wide Web Publishing Service. Still, phpinfo() shows exactly ZERO changes; error_reporting is still set to "22527" and display_errors is still set to "Off," pretty as you please.
So until I can figure out what this boneheaded crap is, I can't implement your suggestions. And quite frankly, this is making me feel like a total idiot.