Thanks for your responses @LostLinuxGuy and @RonW(WDStaff)!
Here’s some answers to your questions, and new information from my further testing, tonight:
NOTE: To reduce detail repetition in this post and thread, I am now informally dubbing this “strange behavior” as the “temp folder bug”. It is characterized by the following symptoms (when I set the Windows temp folders to my desired temp folder (I set them all to the same folder if that matters any)): no application splash screen (instead, the windows cursor icon rapidly changing (between “wait” and “normal”)), process pid rapidly changing, temp folder rapidly filling up with randomly-named folders each containing a ~4MB copy of the stub installer until hard drive is full).
@LostLinuxGuy
TLDR:
Thanks for the tip!
CHANGING THE TEMP FOLDERS BACK TO THE ORIGINAL WINDOWS DEFAULT TEMP FOLDERS FIXED THE PROBLEM (If anyone else needs them, I listed them below)!
However, even in the latest version of the WD Dashboard app, the “temp folder bug” still exists.
SUMMARY #1: It seems like both “dashboardsetup.exe” and “westerndigitalssddashboardsetup.exe” exhibit the “temp folder bug”.
SUMMARY #2: It seems like the Western Digital SSD Dashboard v2.6.0.0 in-app updater is not dynamically downloading the latest installer(?) but is instead hardcoded to download/use “westerndigitalssddashboardsetup.exe” maybe? But I have confirmed on my systems that THE IN-APP UPDATE PROCESS definitely still exhibits the “temp folder bug”.
Read on for DETAILS…
(SUCCESSES)
I changed my Windows User TEMP and TMP environment variables back to %userprofile%\appdata\local\temp
I changed my Windows System TEMP and TMP environment variables back to %systemroot%\temp
I ran the DashboardSetup.exe (v3.0.2.37, release date 2020-09-14) I was using on Oct3 and it appears to be working (I received splash screen, which I had not before!)!
(So I cancelled it to do more testing).
Then I ran the current DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08) I downloaded tonight and it appears to be working (I received splash screen)!
(So I cancelled it to do more testing).
(DEFEATS)
Then I changed my my User TEMP and TMP environment variables and my System TEMP and TMP environment variables back to my preferred temp folder.
Then I ran the current DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08) and it exhibits the same exact “temp folder bug” I originally described on Oct3. 
So there still seems to be the “temp folder bug” in the latest version of DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08).
FYI: To download the latest version of the installer, I go to support.wdc.com, click “downloads”, click “Western Digital Dashboard” and then click the “download” button. On Oct3 and today, this yielded a “DashboardSetup.exe” download.
When I started this topic writeup on Oct3 (with the temp folders set to my preferred temp folder), I also attempted to use the Western Digital SSD Dashboard v2.6.0.0 “New Application Available” Notification link (what you call the in-app update) and the same exact “temp folder bug” occurred at that time.
By the way, to anyone out there experiencing the same “temp folder bug”, you can easily get it to stop by issuing one of two windows commands: “taskkill /T /F /im dashboardsetup.exe” and “taskkill /T /F /im westerndigitalssddashboardsetup.exe”. Other ways to get it to stop: maybe logging off / signing off, or definitely by rebooting your computer. But in my opinion, the commands are easier (I put them in a bat file with a “pause” command as the last command and double click it when I need it). I was not able to use task manager (taskmgr) to kill the rogue process, because the rogue process pid changes so rapidly that you cannot focus on it long enough to click the end task button. (That exe terminates and a new one starts, causing your taskmgr selection to lose focus in the exe to kill).
@RonW(WDStaff)
(please also reference discussion just above for some answers to your questions, other answers below)
I tested, and the current DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08) only works when the temp folders are set to windows defaults.
When the current DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08) is set to my desired temp folder, the “temp folder bug” still manifests.
On my Oct3 writeup in this thread, (with temp folders set to my temp folder) I had originally tried to use the Western Digital SSD Dashboard v2.6.0.0 “New Application Available” Notification link (the in-app update) and the same “temp folder bug” occurred.
I tried it again today while writing this. (With temp folders set to my temp folder) I tried to use the Western Digital SSD Dashboard v2.6.0.0 “New Application Available” Notification link (the in-app update) and again, after it (seemingly) downloaded something from the internet (with a progress bar) for about 30 seconds (presumably the stub installer) and then I pressed Ok when it prompted, the same “temp folder bug” occurred.
AN OBSERVATION ABOUT WHICH EXE IS BUGGING OUT:
SUMMARY #1: It seems like both “dashboardsetup.exe” and “westerndigitalssddashboardsetup.exe” exhibit the “temp folder bug”
SUMMARY #2: It seems like the Western Digital SSD Dashboard v2.6.0.0 in-app updater is not dynamically downloading the latest installer(?) but is instead hardcoded to use “westerndigitalssddashboardsetup.exe” maybe? But it definitely still exhibits the “temp folder bug”.
When I killed the Western Digital SSD Dashboard v2.6.0.0 in-app “temp folder bug” with my taskkill bat file, I noticed in the console output that there was no “dashboardsetup.exe” process, only “westerndigitalssddashboardsetup.exe” process:
((
taskkill /T /F /im dashboardsetup.exe
ERROR: The process “dashboardsetup.exe” not found.
taskkill /T /F /im westerndigitalssddashboardsetup.exe
SUCCESS: The process with PID 12116 (child process of PID 8568) has been terminated.
))
When I killed the current DashboardSetup.exe (v3.1.2.5 (Installer 4.0.2.0), release date 2020-10-08) “temp folder bug” with my taskkill bat file, I noticed in the console output that there was no “westerndigitalssddashboardsetup.exe” process, only “dashboardsetup.exe” process:
((
taskkill /T /F /im dashboardsetup.exe
SUCCESS: The process with PID 9712 (child process of PID 13228) has been terminated.
taskkill /T /F /im westerndigitalssddashboardsetup.exe
ERROR: The process “westerndigitalssddashboardsetup.exe” not found.
))
When I killed the Oct3 DashboardSetup.exe (v3.0.2.37, release date 2020-09-14) “temp folder bug” with my taskkill bat file, I noticed in the console output that there was no “westerndigitalssddashboardsetup.exe” process, only “dashboardsetup.exe” process:
((
taskkill /T /F /im dashboardsetup.exe
SUCCESS: The process with PID 9712 (child process of PID 13228) has been terminated.
taskkill /T /F /im westerndigitalssddashboardsetup.exe
ERROR: The process “westerndigitalssddashboardsetup.exe” not found.
))
REPEAT FINAL SUMMARY:
CHANGING THE TEMP FOLDERS BACK TO THE ORIGINAL WINDOWS DEFAULT TEMP FOLDERS FIXED THE PROBLEM (If anyone else needs them, I listed them above)!
However, even in the latest version of the WD Dashboard app, the “temp folder bug” still exists.
SUMMARY #1: It seems like both “dashboardsetup.exe” and “westerndigitalssddashboardsetup.exe” exhibit the “temp folder bug”.
SUMMARY #2: It seems like the Western Digital SSD Dashboard v2.6.0.0 in-app updater is not dynamically downloading the latest installer(?) but is instead hardcoded to use “westerndigitalssddashboardsetup.exe” maybe? But THE IN-APP UPDATE PROCESS definitely still exhibits the “temp folder bug”.
By posting this much information, I hope it can help someone else with the problem or even WD programmers to find and fix the “temp folder bug”!
