[Samsung Galaxy S8 Guides, News, & Discussion] serendrewpity:
I think my issues are resolved. I'm posting this in the hope it will help someone else but also for myself so I remember what I did in the future if a new root process is not found. I apologize in advance for the length.

So, I did try the Prince Comsey odin. I also tried Odin 3.07, 3.10.6, 3.12.3 & 3.12.7. The instructions for creating a custom TAR Ball for flashing involved using a version of odin that had a 'Phone' button that was on older versions of Odin but on newer versions including PrinceComsey that button was renamed. No version was successful though. Or so I thought...

In the end, the second half [Part 2] of the SamPWND instructions call for you to use Flashfire to flash the stock system.img.ext4 and SamPWND_G950U. In one of my attempts, I unsuccessfully tried to get Flashfire to flash the custom NON-HLOS.tar.md5 file along with the other two files. I say it was unsuccessful but it is really hard to tell what's going on while Flashfire is doing its thing because the screen goes really dark and while you can see something happening, you can't actually read anything.

Anyway, in the process, I made a decision that I would have Flashfire NOT reboot back into Android [bootloader] but reboot into 'Download' mode immediately after flashing system.img.ext4 & SamPWND_G950U. In download mode I then used PrinceComsey Odin [which was still open from Part 1 of SamPWND instructions] and flashed the CP & CSC archives mentioned in the OP of this thread. I tried this a couple of times. Even though the flashing of NON-HLOS failed each time [or maybe it didn't; unclear], I noticed that after flashing the CP and CSC files with PrinceComsey the random reboots had stopped. I couldn't seem to recreate the 'Subsystem Crashed: Modem!' error and I was really trying.

So to make sure I understood what was going on, I started from scratch. I used Full Odin 3.12.7 to flash Samsung Galaxy S8 Original Stock firmware. Made sure that stock firmware and the phone's hardware were still functioning as expected. It was. I then followed the SamPWND instructions to the letter. Including using only PrinceComsey Odin. The only thing I changed was the very last step I had Flashfire reboot the phone into Download mode (without booting into Android) and then used PrinceComsey Odin to flash the CP & CSC archives as mentioned in the OP of this thread. This seems to have worked for me. I have manually rebooted serveral times. I have gone out to various stores and different WiFi environments that I know caused the Subsystem Crashed: Modem! Error.

I have been running for about 18hrs without any errors at all. I will test some more tomorrow but I have a good feeling about all this.

In the end, with my phone [Sprint Galaxy S8 (SM-G950U) - Snapdragon variant], I never had to use the NON-HLOS.bin file or the custom TAR I created. I only needed to have Flashfire avoid booting into Android and instead reboot into Download Mode as its final step and then use PrinceComsey Odin to flash the CP & CSC archives mentioned in the OP.

Note: Be Careful of other versions of the Samsung Galaxy S8 Root (Snapdragon) Instructions you may find on the internet. I ran into a couple that were wrong and caused me major headaches. Only use the Samsung Galaxy S8 Root (Snapdragon) instructions found on XDA-Developers. :victory: :cowboy: :fingers-crossed:

---------- Post added at 05:58 AM ---------- Previous post was at 05:44 AM ----------

By the way I used SamFirm_v0.3.6 to obtain the latest Samsung Galaxy S8 Sprint firmware directly from Samsung servers using the instructions located at the link below... The CP, CSC and System.Img.Ext4 files were obtained from the same download so there would not have been a difference in the versions

"How to download Samsung Galaxy Full Firmware Files Directly from Samsung Servers"

(I can't post external links, but google the above string of text and look for the site called, androidsage.com)