I've noticed some weird things about this scenario.
1) Let's say you try to close the Chasm of Shadows by defeating a summoned Demonic Platoon henchman, but you fail. I guess, since the Demonic Platoon is summoned, it goes back into the box and is not shuffelled into a random open location deck as indicated on the card. However, the Demonic Platoon isn't used in any other AD2 scenario, and the Afghanistan Principle prevents it from being used in any later scenario, so what good is that provision about shuffling it into a deck on the card? Is it ever used at all?
2) Am I correct in assuming that if, for some reason, the Chasm of Shadows is the last open location, the scenario can no longer be won? You cannot defeat Eustoyriax as long as the Chasm of Shadows is open, and you have no way of closing it.
3) If you fail the scenario, are you supposed to keep the Banner of Valor? It's weird for loot in that you get during the scenario, not at the end.
1) The extra text on the Demonic Platoon is likely for people who want to use the card outside of the standard AP scenarios. It's similar to the villain in Here Comes the Flood from Rise of the Runelords. Also, the Demonic Platoon shows up in the organized play scenarios as a regular henchmen if memory serves me.
2) You are correct. Thus, it's definitely a good idea to close the Chasm sooner, rather than later.
3) It's odd, but I'd say that you keep the banner. As you said, you earn the loot during the scenario. Whether you win or lose doesn't seem to matter.
Regarding 2) I noticed that the scenario also includes the Abyssal Rift which is never permanently closed. So it should be possible to encounter Eustoyriax in the Chasm of Shadows and having the Abyssal Rift on the open side. The villain will then flee to the Abyssal Rift and the Chasm can be permanently closed by exploring it until it is empty, or temporarily when Eustoyriax is encountered at the Abyssal Rift. Complicated, but not unwinnable.