[brazen1]

I'm not sure what you're using the Task Scheduler for because I didn't see any need for it.

Because after you reboot Windows, you will see it enables stereoscopic even though you turned it off prior to rebooting.
So, you have task scheduler turn it back off for you.
As I wrote, in your case you want stereoscopic off at all times otherwise 2D is going to play in 3D. This is why you need to manually enable stereoscopic just prior to playing a 3D title, either from the Kodi GUI or mapping your remote to toggle nvstlink.exe.

There is 1 thing and only 1 thing which changed between all of those flawless 6 years and last month: nVidia GTX 1080.

Windows 10 wasn't released until July 29, 2015. That changed. Yes, it makes a HUGE difference in setups. You have to determine your hardware/software and use your best judgement in all these posts to set something up for yourself. Everyone has different combinations.
Are you using:
1080 or 4k?
W10, 8, or 7?
Smart or dumb hardware?
Which Kodi build?
There is a different method for all of these variables. I don't think there is difference in Nvidia models. I used a GTX 650 and now a GTX 960 and both work. I don't think your 1080 would be any different.

I can read your frustration. Been there done that. Eventually I worked it out using the various methods in this thread. As time goes by, so does O/S upgrades, hardware, etc. thus why there are so many methods to accomplishing..... and the confusion. Toss in user variances and here we are. I'm not dissing your PJ or telling you to upgrade or anything. I can tell you this so you understand. Was running the same setup for years just like you. Upgraded to W10, had to make a bunch of changes. Worked for a year perfectly. Upgraded to smart 4k and had to make a bunch of changes. I could literally swap the HDMI cable from dumb 1080 Samsung panel to smart Samsung panel and Nvidia went crazy. It was then I learned ALL the mods to make dumb 1080 and Nvidia work were because my dumb 1080 was inhibited. The moment I plugged HDMI from Nvidia into the smart panel, all those mods went out the window because everything just worked in harmony with 4k. Thus, Nvidia and it's drivers are not a problem. The problem is forcing Nvidia to continue playing nice with older hardware and forcing it to do what the hardware cannot. This is why this thread has so many variables. One of the methods will work. We are trying to diagnose what does not work, why, and how to 'fix' it.

Does it matter if you play 2D media or 3D media? Or should I play one or the other & then hit 'Yes' on the match refresh rate?

Doesn't matter, once it understands 24Hz, it's set.

Do you leave Stereoscopic 3D "Enabled" or "Disabled" in the control panel. In a previous instruction you had mentioned Disable it after making all the adjustments. However, in this post you didn't mention that. Just wanted to clarify.

In your case, disabled. You always want it disabled, thus the task scheduler and nvstlink.exe toggle.

We are trying to establish a 100% working setup starting from scratch. If you are integrating codes from elsewhere, modding what's here, etc. I can't help you.
Although it's cumbersome, I'm fairly certain if you follow my instructions, you will eventually get everything in harmony. Don't assume or cut corners. Once you have a working setup, mod from there. You tried method 3 from scratch following to a 't'. Okay, it didn't work. As, stated, you would be testing that one. Now I understand the consequences of what went wrong. 2D as 3D etc. Nvidia thinks 24Hz is 3D. Your PJ won't tell it otherwise. Time to intervene.

You say you already tried:
Since a 2D is switching to 3D, then you have to control Nvidia nvstlink.exe using the original setup post #1 with both 2D and 3D .bats and playercorefactory (post #3) for non-smart.
You will also need to setup task scheduler post #46 because W10 is going to turn on stereoscopic at reboot and you want it off.
Nvidia stereoscopic will always be off and everything at 60Hz only changing to 24Hz during playback.

Again, don't assume or cut corners or copy old files etc. Follow the instructions fresh exactly and let's see what you end up with.