So gonna preemptively leave a comment as I gather the things I think the authour is signaling is a clue.
So following the logic Tsama0 is giving for this being interconnected in some way I think I already have a good idea on how this is linked, with Mystery A being the overarching story and Mystery B being the focus story here are the things I noticed that I think are relevant as clues, that I feel the authour is pointing out. The actual mystery hasn't been pointed out yet but I think we can get a pretty good guess that it is related to the new single mom MC being stalked.
-The authour goes out of his way to point out the stencil ruler
-Mentions twice that the MC is wearing makeup as well as having a character point out it is heavy makeup (even in a work setting)
-The new MC states "I'm being followed again"
-The MC is widowed.
-This one might be nothing but I've noticed that the MC is kind of uniquely tall for a woman (but I'm not too sure about this point), we get a measure of her height in 3 instances against another adult. With both women she is next 2 she looks significantly taller however with the man she appears kind of average height, so this might just be nothing.
-Finally the core picture mystery, the most immediately noticeable thing is the huge smudge. Honestly, not alot to work off of right now but will jump back on this when I think of more stuff.
So to leave a crackpot theory for Mystery A that almost solely hinges on 3 clues, 2 of which is pretty shaky. Going to leave a spoiler here since this would be for Mystery A. I think the current MC is actually Ren from the past mystery. Here is what I'm grounding the theory on; the MC is pointed out to be wearing heavy makeup which could be the case to help hide their identity, the MC I think is pretty tall for a woman, and finally the picture of the husband. Specifically if we look at the picture of the husband he has the exact same haircut as the silhouette we are given of Ren, additionally the opening scene of the chapter is what appears to be a man with a young child visiting a grave, and not a woman which would tie into this mystery but ties very well into what we know about the previous mystery. Additionally, this links into the unknown crimes that Yuki committed as a reasoning for why Ren would need to go into hiding and change his identity.