I think there were a couple of other things that were argued by some as hints towards Actor content, but from my current state of distress, I can't remember. Do let me know?

The most likely is that this is a combination of meaningless bits and just coincidences, however...

I bring your attention to the Wilford "Motherloving" Warfstache explanation livestream. There, Mark said that the "Oh, I remember the train!" line was just a reference to Murder on the Orient Express, with the intent to communicate the characters' new status as, well, characters by providing a representative piece of media with roles they could fill.

However, then — months ago by now — we got the Train Project teaser. It's widely accepted by now, although still theory, that this project will be essentially the backstory for that line — some events on an old-timey train with the egos; another project with Akupara Games. (Relevantly, even this on its own had some people wondering which versions of the characters we would see, and if it would be "true Actor content" vs something like Heist which is often interpreted as the Actor playing a character.)

In this case, though, that's just to show that— Mark's changed his mind before, and decided to take something from "that means nothing!" to an actual piece of content.

Assuming the theory is correct, of course. That's all we can say about this first part — theory, speculation.

If we look at the post on Instagram, however, we can discuss something arguably more interesting because it is concrete.

This... makes YouTuber!Mark = Actor!Mark canon. After all, Mark’s posting from his own account, in the first person, but obviously this is the Actor. (To be explicit: Note the video itself with his robe and its callbacks to the music of WKM, and the direct references to the events of the MCU in the caption itself.) And, if you’d prefer to look at it in the other direction, this is the Actor doing a Cloakbrand promo = the Actor is Cloak’s Markiplier = that’s our Markiplier.

And if you do take that as canon, it has some iiiiinteresting implications.

I could talk about the vast quantity of pain it causes me, as an Actor simp, but I will not,

Instead! Zoom in on the line “...’metaphorically’ stabbed in the back about 37 times by people I THOUGHT I could trust.” Once you’ve recovered from the heart wrenching reminder of canon, you can take this at least two different ways.

1) The Actor not only blames presumably William and Celine for what happened, but blames them to such an extent that he doesn’t even view the action (the stabbings) as an action that was taken by him. That’s beyond “it was their fault” and beyond “they drove me to it,” that’s an implication he views it almost literally as their doing. Characterization-wise, this ties right in to the delusion he shows in the Damien animation — his anger and hatred, even more intense than one might think from WKM alone.

2) This one, I’ll admit, I notice mostly because of my own headcanons/the interpretations I use when writing fics. But, the other way you could take it is that the Actor counted the Entity — which is canonically implied to have had a much more direct role in the Actor’s pre-WKM decisions — among the people he thought he could trust, among the people who betrayed him.

And in either case (also, both could be true, I suppose) it confirms that the present-day Actor still hates at least Celine and William, and has stuck 110% to his delusions that he was the victim, that his actions were justified, that he doesn’t regret it, that where they ended up is in some way right or good.