I fixed my own problem. Here's how I did it, in case anyone else encounters it.
1. I used Romaster tool to root originally, and so I figured I'd play with it some more. I started it with Kindle attached and stuck black-screen and it offered to install some drivers. I accepted and it put in a Romaster ADB driver (visible under Android heading in Device manager). However, that driver wasn't useful to me so...
2. I went back to my Android SDK folder and found that in extras\google folder there is a generic ADB driver from Google. I uninstalled and deleted my Romaster ADB driver, rebooted windows, and when it came back up, it showed a device under Android heading with a yellow triangle. I selected my 'All devices' and then 'have disk' and pointed towards that google folder and chose that ADB driver.
3. It installed and then I was able to run adb commands from the sdk (i.e. in the platform-tools folder). Namely, I needed to do adb devices to double-check - and it showed my Kindle. Then, adb shell, su, and chmod on the build.prop to get it back to proper permissions. This has already been mentioned in other places so I won't repeat it here.
4. Then, a manual reboot by holding down the power button and I was back in business!
The point of the story is if you goofed up like me and can only get the grey Kindle logo and then blank screen, you can recover by using the above steps. No need to return your Kindle to Amazon or anything like that. Hope that helps!
Hit a little (or big?) issue myself: tried to edit build.prop, no go - got access denied error. Figured I'll give Kindle a reboot, but now it's stuck. It shows the grey Kindle logo and then the screen goes dark (although slightly backlit) and just sits there. Any thoughts? Maybe build.prop got goofed up somehow and that's causing the booting issue?
Clarification: when I was messing around with build.prop I changed it's permission to writable (by all), so maybe that is what's causing the trouble. If I reboot my Kindle with USB attached, it says something about not finding MTP drivers.