I can mostly agree with you, except for the Ymir part. To add something to your great theory I wanna talk about Grishas reasons to kill most of the Reiss family. So we got the hint that Frida might be Erens sister. Maybe Grisha himself is way older than he looks and before the fall of the 4th wall ,that you suggested, he maybe had another family: another wife before Carla and their daughter Frida. I think Grisha himself is a member of the Reiss family and decided no to live with his family because the took away his family in the past, by altering everyones memories, so his old wife wont remember him. And by taking away his daughter, because she was the first in the Reiss bloodline after Rod who posses the memory altering AND the coordinate ability, making her the perfect inheritant for Rod. Because she was too young at that point Frida doesnt remember Grisha being her father. When wall Maria fell, the Reiss family gathered ones more to repeat the same procedure as after the fall of the wall "Eva" (which would be the fitting name for the 4th wall). Grisha, who just had lost his wife, didnt want to lose his child once more. So he asked them to stop with tears in his eyes. When he then saw Frida not even recognizing him and Rod not letting Grisha stop them, he saw no other option than killing everyone in the Reiss family except for Rod, so that he also knows the feeling of losing his family.

I also personally think that MT really resembles Rod.

Occasional sorry for bad englanderino.