I think they are the real Survey Corps. It's just that it was shown that they returned as it was also told how they are in the basement. Kind of simultaneous. Since the Beast Titan and the others have backed off for now it is safe for Heichou and everyone else to return. This is a pivotal point as that revelation of humans living peacefully (?) outside of the walls is now a verified truth, we'll see how Pixis and the others in power will do about this fact. It's like the sole focus won't be on exterminating titans only but looking for that existing life from the titans. The chapter brought more questions due to that revelation.
The legacy of Erwin will always remain on the Scouts, especially on Levi, Hange and the rest. Yes it is sad that he left before he can verify the truth but Imthink within himself he believes his theory is right. :)
I hope you are right that they are the real survey corps... if not, thats a big trouble! And i agree, that too many questions appear... and thats exciting and little bit frustatrating.. XDDD
Why there is no one talking about the page where they said "the survey corps has returned" ?? when all of them are still in wall maria... i think they are imposters.. base on the height and built they look like levi, hange, eren and mikasa (because of the scarf) i think they will cause trouble and when the real survey corps return, they will take all the blame... did i misundertand that page?? (⊙…⊙ )
And it is so sad that erwin really died... all this time, his theory is correct, no one believe him, and he didnt know that before he died... :(((