SOLUTION:
- Connect external DVD drive to Macbook.
- Insert Windows 10 Pro 64-bit installation DVD into DVD drive (does not need to be the same disk you used for the original install).
- Cycle power and hold down OPTION key.
- You should then soon see a screen showing two "drive" icons, corresponding to the MacOS and the Boot Camp (Windows) partitions, allowing you to choose which one to boot. WAIT. After a short delay, you should see a "DVD" icon appear, corresponding to the DVD that's present.
- Select the DVD icon (Windows Installation disk).
- After some delay, a Windows dialog appeared, asking me to make 3 choices regarding installation language and locale, keyboard language, etc. Since I don't want to (re)install Windows, I canceled this dialog.
- After a small delay, my computer showed me the standard Windows 10 login screen (displaying a random background photo and the current time). I was able to log in normally (and was greeted with a notification that updates had been installed). So I ejected the DVD, disconnected the external DVD drive, checked for and installed Windows Updates, and restarted.
Now the computer boots normally. So apparently the process of booting from the installation DVD, even though I cancelled the installation before it ever started, was enough to reset the OS into a working (bootable) state again.
NOTE: Before restarting, you may want to take the time to make a Recovery USB stick (it's very quick if you don't choose the option to copy system settings and data), and/or to enable the legacy boot menu on F-key (to help in future scenarios), find and write down your Windows Product key, etc.
I hope this helps someone!