My observations from my journey (Fastboot only, Bootloader Locked):
1) made sure my Windows computer was in test mode (powershell... bcdedit... find the instructions on how to do this). SUCCESS!
2) Phone kept going into Fastboot mode whenever plugged in -> found a thread that mentioned forcing it off by holding Power + BOTH Volume buttons (10 seconds). SUCCESS!
3) got Qualcomm drivers to recognize phone when plugged into PC = allowed my device to be seen from MSM tool (along the way, downloaded and used both OP drivers in 1st post - maybe it helped, maybe not). SUCCESS!
3) In the MSM tool, I would get the "sahara communication failed". Unplug phone and plug in, MSM starts again, but gets another "sahara" message. Repeat dozens of times, to no avail. FAIL!
4) Was using the OOS 9.0.13 file and MSM tool, but read a thread that suggested using an older set, so I switched to using the OOS 9.0.5 file and MSM tool. Get another "sahara" message FAIL!
5) Followed some suggestions to hold volume up and power button (not sure why). Phone goes into Fastboot mode, and the phone would only register as Android Bootloader Interface when Fastboot was on. Could not upload any other driver for it to work. FAIL!
So: The phone is in Fastboot mode, is still plugged into the computer; the MSM tool from the 9.0.5 tool is still open, still set on "Sahara" error message from before; I have loaded both Qualcomm drivers the OP has listed in post #1, yet the phone still reads as "Android Bootloader Interface" and not Qualcomm when in Fastboot mode. FAIL! FAIL! FAIL!
6) Turned off phone by holding down Power + BOTH Volume buttons (because anything else forces the phone back into Fastboot mode because it is plugged in!) and suddenly the MSM tool starts downloading the appropriate files to fix my phone. SWEET SUCCESS!!!!!!
This post is mainly to help myself in the future should another brick happen, but to help others as well learn from my mistakes. I read a lot of posts and started inching closer and closer to my ultimate goal of full stock restoration - thank you to all at the XDA community and Youtube as well. But this post is also to illustrate that not everyone's situation is the same, and it can be incredibly frustrating when your situation does not fit the bill.
Thanks to everyone, and good luck with yours!