Given the "shot dead", Zhao Xinci is almost certainly responsible, though it might not necessarily be during the case where Shen Xi died. Your interpretation is disturbingly plausible, though, and I'll see if the show comments upon it in later episodes.

Shen Wei outright says "twenty years" in one of the conversations he had with Wu Tian'en. (This is in my first watch notes rather than rewatch notes, so I don't have the scene.) It's possible to mentally judo the timeline into other shapes, but those all require interpretation and selectively ignoring some pieces of canon. (My solution is hilaribad, but sort of incorporates all the canon?)