I will continue to document my findings in order to hopefully help the next person who encounters the same issue.

I watched over the video I posted above again, this time also referencing a diagram in the repair manual:


From 0:04~0:06 in the video, I realized that the trunk actuator motor does seem to be operating incorrectly, according to my understanding of the diagram at least.
It rotates from the "close completion position" (locked, pic 1 below) to the "open completion position" (unlocked, pic 2 below - which is where it should also stop I think), but then rotates back to the "close completion position" again (locked, pic 3 below).





P.S. now that I've located that the issue comes from the "soft close" function, I did some more digging and found another user with the same issue as me here. Unfortunately they couldn't fix the problem by replacing the actuator either...

 

Now I have two ways to try and solve this problem.
The first is a macgyver solution where I just order a custom set of high-pressure trunk struts, then add some some extra stiff springs to slot onto the end of the trunk struts so that the trunk will hopefully spring open under the tremendous pressure whenever I unlock it, before the actuator even has time to lock it back down again. The second is to start diagnosing all the electrical connectors according to the specs listed in the repair manual to locate the point of fault and fix that, which will probably be a big headache...
For the meantime I'll try the first method, and will be diagnosing the electrical part while waiting for the parts to arrive

 

Did you try lubricating the trunk hinges? Do you have anything non-standard attached to the trunk lid that might be weighing it down?
It sounds like when you release the trunk it is not lifting quite enough, so the Soft Close is being triggered and it is closing the trunk before you get a chance to lift it.

 

I will try to take some WD40 to the hinges, though I doubt that will do anything since the trunk itself isn't exactly hard to lift when it's unlocked. Nothing attached to the trunk lid.
I wish that was the actual problem, but from my understanding the trunk actuator goes through the "unlock signal received - unlock phase - soft close activated - locked again" ALL the time; even when it's fully open you can see the actuator performing the "soft close" phase, hence the second whirr of the motor (as you can see in my video). At least that's what I think it is, I tried looking all over the internet to find a good quality video of the trunk unlocking normally to find the "correct" actuator motor noise, but no luck, so I can only base my findings on what I observe on my own car.

 

Ok, if the latch mechanism is cycling constantly then something is broken.
It could be that the soft-close trigger is stuck in the triggering position.

I'll see if I can get a video of mine with the trunk lid open for ya.