In Attack on Titan, why did the royal government put King Fritz on the throne? And how did Erwin and the Survey Corps know that he was a fake king?
Why did the royal government put King Fritz on the throne? It was never stated directly but as seen throughout the revelations in the manga, the former king Karl Fritz always wanted to erase any memory and information about Titans and the outside world. As stated here,
Fritz held the belief that the world would have been better off if Titans and Eldians never existed and he considered his people to be sinners.
In Chapter 66, Fritz's descendants who inherited his Titan also inherited this belief of a world ruled by Titans as 'true peace' and as stated here,
this influence is so strong that even people that wanted to stop the Titans changed their minds when they received the power, deciding not to make a move and refusing to reveal the reason, even to their own families.
This, to me, is the reason why this led to the placement of the fake king: aside from preventing Eldians from discovering any information about anything outside the walls, this was also to protect the identity of the Titan shifter within the royal family. If the people within the walls found out about the Coordinate and the hostility of other nations, they would, naturally, want the Founding Titan to defend them. This goes against Karl Fritz's wish, who was ready to accept punishment with arms wide open. This will is inherited by his descendants who hold the Founding Titan and as stated above, they are forced to accept this will whether they like it or not.
How did Erwin and the Survey Corps know that he was a fake king? In Chapter 55, Hannes and Levi were torturing Djel Sannes from Military Police Brigade who they captured. Sannes himself stated that:
The Reiss Family is the true royal bloodline.
Erwin was then shown receiving the information from a messenger.
The true king needed someone who would take the throne so Rod and his family could not be in the center of attention/ be protected from assassination attempts so they could keep the Founding Titan in the bloodline, and so they cont die and have the titan fall into Marley or some person in paradise.(speculation on my part). If I was King Riess I would do exactly the same thing to keep the things above from happening. Assassination sucks, after all that work and effort to keep the Founding Titan out of Marley's hands? its no wonder there was a fake king.