Which "guide" are you talking about? I only see in Post #1, the troubleshooting tip to flash stock boot.img, which is only the kernel (which is not the proper process to install recovery image). Unless you are talking about a subsequent post on this thread (I didn't look at all 15 pages!).
There are instructions on the NVIDIA download page where you get the stock recovery images. You need to flash system.img, boot.img and staging blob (all flashed separately in fastboot) in order to properly restore to stock. Missing any one of those, will likely cause the stock ROM to not boot.
You actually don't need to flash the recovery partition itself (even though the NVIDIA instruction say so). I've successfully restored the rest of the stock image (system, boot, staging blob), but purposely skipped to step to flash recovery.img (keeping TWRP).
If you want to get TWRP backup just fastboot flash recovery TWRP.img
The issue is that this one is one of the recalled versions. It works are you suggested with the stock NVIDIA rom (Removing the OTA files before connecting to the net.), but I don't trust this fix.
1. Issue with TWRP..
The latest Nvidia Shield Tablet version from TWRP site does not work after installing via ADB. The only version that works is 2.8.7 which wasn't from the official TWRP site. In fact all the versions from the official site fail to work on my Shield, I don't know why! I'm starting to think there is something wrong with their files.
2. Issue with Customs Rom..
Using the only version of TWPR that works (2.8.7) I've attempted to install Bliss, RR, Omni, CM and Lineage. All fail to install due to the first 2 line of their 'updater-script' file. The lines that start with 'assert'.
I've read that I needed to delete these lines with Notepad++, so I did. The installs seemed to work, but failed to write anything to the Shield! So it fails to boot.
So you see I'm at a loss with what I need to do next.
I guess install the stock NVIDIA rom and pray the fix works!