Responses
I'm not sure about the when (I mean if he been in there longer), but I think he got into Marley a few chapters ago when Gabi pulled her stunt in battle and became a hero. He joined the ranks of the returning, injured eldian soldiers suffering from PTSD. I don't think in the chaos of the battlefield anyone would have payed attention to one more injured "eldian dog", so he could get in unnoticed.
Dude straight up managed to infiltrate Marley. How the hell did he do that? Especially with how strict the Eldia concentration camp security. Also since when? what about others like Mikasa and Armin? Did they also become spies?
More chapters showed up, more questions popping up.