[Fire Q&A, Help & Troubleshooting] Blastyrant:
WARNING Do not downgrade below 5.1.2. It has been confirmed to brick the device.

Here is a status update for anyone who is attempting to do the same thing and has been running into obstacles.

I uninstalled all of the drivers. I did this with a system restore. But you could just as easily do this manually. I've also seen some "wipe" tools online that claim to rip out all Android drivers, maybe try one of those if needed? Once they were gone I turned the Fire 7 ADB debug mode on. I then installed the Kindle drivers from Amazon. Then I plugged in the Fire 7 via USB and let it "install drivers". I checked Device manager and this time ADB was showing up. I then shut down the Fire, held volume down+power to boot into Recovery Mode. I had to then manually point the Fire in Device Manager to the correct drivers. Then I sideloaded the 5.1.2 firmware and it has flashed successfully. Next is Root and then CM12.1.

Here is what I did:

To start, it was running 5.1.3 out of the box. I never connected it to WIFI, never altered it, never updated it, etc. It was a stock Fire 7 Gen 5.

1. Delete all previous Amazon driver installations (if required)

2. Install THESE drivers (I'm on Win7)

3. Enable ADB debugging on your Fire tablet (Info can be found here)

4. Download and extract THESE ADB Fast Boot Files from Root Junkys.

5. Download the v5.1.2 OS build from HERE and then verify the SHA1 and MD5 checksums with THIS free online tool.

SHA1: 66B5423725B79CEB0D5866FA32FF414A99A4B50A

MD5: 5B0DCC957DEF3ACCFF58BF33794D6D88

6. Connect your powered-off (turned off, in the off state, etc) Fire 7 5th Gen tablet via USB and then hold volume down+power to boot into Recovery Mode.

7. (perhaps optional) If Fire is not listed as a usable device in Device Manager when in Recovery Mode (or has that warning icon next to it), you'll need to manually point it to the drivers you installed.

.....Select Other Devices > Fire and then Right Click it and choose Update Driver

.....Click the button to let you pick from a list of drivers on your computer. Find the Amazon device (Fire Device) in the list and then choose "from disk".

.....Navigate to the default driver install folder (C:\Program Files (x86)\Amazon.com\Kindle Fire\Drivers)

.....Select Android Composite ADB interface (this is for Stock ROM, recovery/adb sideload)

More info for step 7 can be found HERE under "Connecting Your Tablet to Your Computer"

8. Hold Select and Right Click anywhere in the extracted ADB Fast Boot Files folder and choose "Open Command Window Here" from the context menu.

9. In the command window type "adb sideload <path to the v5.1.2 firmware>"

.....IE: C:\Users\MyPC\Desktop\Amazon Fire\ADB FASTBOOT FILES WINDOWS>adb sideload "C:\Users\MYPC\Desktop\Amazon Fire\update-kindle-global-37.5.4.2_user_542168620.bin"

.....You can make this easier on yourself by typing "adb sideload " and then drag-drop the v5.1.2 file into the command widow and the correct path will be automatically populated for you.

.....Click Enter, and allow the process to run its course

You should now be running v5.1.2 stock firmware, which is fully rootable. Next up is rooting and then installing CM 12.1.

I'll start reading up on rooting from THIS thread. I'll make a post with what I did after I have it working.