So Porco and Pieck are now trapped underground because of a soldier misdirecting them but its kinda hard to make out who it is as the panel only shows half their face. However I'm now starting to suspect the soldier in question is Jean Kirstein. Pieck said she felt like she'd seen the soldier who'd trapped them before. In the battle for Shiganshina, Pieck as briefly encountered Jean a Titan while rescuing Reiner from Hange and as far as I recall he was the only soldier she encountered besides Eren and Levi who survived the battle as she spent most of it at Zeke's side. Speaking of whom that same soldier in question told Zeke to be elsewhere just before trapping Porco and Pieck. Zeke was with Pieck when she rescued Reiner so he too might've recognised Jean if he is the soldier in question. I also believe Eren and Zeke are working together so he might've known what was going to happen.
So yea that's my opinion but I really am starting to think the soldier in question is Jean and I would be thrilled if that is the case. Jean has always been one of my favorite characters in this series and I think this would be an awesome come back for him and Eren to be infeltrating Marley together when the two couldn't stand each other to begin with.
Maybe Isayama just felt like making him that tall. Also, Pieck only saw roasted Armin, she wouldn't recognize him after he was healed.
I doubt Armin is even in Marley, since they wouldn't leave the Walls without a Titan. Even Marley decided to leave Beast and Cart Titans on the continent as deterrence when they sent the rest of the warriors to the island.