On a really crazy note, just thoughts that came out as I binge read all of this, could Uichi of an alternate "successful" timeline be the mastermind? In like a really twisted way, say if in that time line where Yotsuha saves him from his demise and brings him a better life right, but it comes at the expense of her own life. Having overworked herself to succeed in accomplishing this outcome she goes back to have the heart surgery too late, or not at all and she dies as a result. Perhaps for one reason or another she had admitted all that she had done to Uichi, and/or he ends up in similar turmoil over her death he too end up getting the ability to travel back in time. Unable to live with the thought that he would ultimately be the very reason he one he loved would end up dying, he orchestrates this whole plan, knowing just the right people to push towards malicious actions against his alternate self. Not necessarily thinking about what that might do to her as a result merely just wanting her to be alive regardless of the consequences.
Now since we also don't have any real reason to believe two of the same person can exist simultaneously, or at least not through the same method like the watch, the idea of alternate version time travelers is a hard to pass idea. So don't currently believe this is an actually plausibility, but it was a fun thought experiment I wanted to share.
Loving this story so far, but ever since her reveal of her heart condition, I am scared this isn't going to end well. We have no way of knowing waiting that long for the surgery is good for her and will even work, or if it does work, won't have sever consequences. This story concept of a kinda reverse tragedy is very fun to read though and hoping it continues that through to completion and ends happily. Undoing all the damage and tragic events to hopefully create a happy future, and they aren't being too crazy with the time travel stuff that it's hard to follow or creates logic holes so great work on that.
Oh also, I don't envy her, though I heavily admire her resolve to push through all this trauma and have to live without telling anyone, that she has essentially seen the dead body and even death of the one she loves who knows how many times. Can say I'd probably try for the same if I was directly in her shoes for someone I loved so dearly like that, but to go through and live with that, she is super smart, and very dedicated. The emotional bomb of all this though I imagine has to detonate eventually.