I was grasping for straws this whole time, searching for something that didn't exist, a secret emulate mode.

In my last post I looked at the official hardware debug tool and I was just overthinking it, it's just protection features. While going over the datasheet, it seemed the only issue preventing my design from working was the first byte that can't be set.

Looking in GPD, it’s a LUT. I'll try using another LUT and see if I have the same problem. Now the design seems to have been written to the chip but it doesn't work, I checked the connections in GPD... I've missed a connection to one of the timers...
The test design works now. Holding the button down for 3 seconds turns the LED on; holding it down for 8 seconds turns it off.

Now let's go back and use the LUT I did before. It does not work. Is there a problem with my code? The first byte does not write properly? I'll have to go through my code and perhaps write some tests.

I've not wasted any time though since I've learned a lot from this. I wrote a lot of code to test all kinds of stuff. I looked more closely at the protection features of the SLG46826 and got a better understanding of how it works.

I do feel a bit stupid for assuming that emulate mode was a secret, I also made faulty assumptions about all kinds of things.

As with all other mistakes I've made, I learned a lot. ;)