Wrt the dead baby theory, I suspect that if that is the case then it has to have been Bella's fault in some way, not just a simple miscarriage.
Now, that's not saying she smothered the kid in their sleep or something like that. Most likely, what happened was an accident. But Bella clearly blames herself for whatever caused James to leave. And while the dynamic of "baby dies, man leaves, woman blames herself" is nothing new - that's actually how my aunt's first marriage ended - I don't think that's quite the case here. Much of Bella's identity is built around being level-headed and reasonable, so I think she would, with time, accept that in that case there was nothing she could've done and that sometimes bad things happen to good people.
But no, even years after James left, Bella still blames herself. It's possible she simply never recovered from the grief of her family collapsing, but something about that just doesn't ring true for me. Call it a hunch. I believe there's a specific reason she blames herself - and, if the dead baby theory is true, the obvious answer is that she was, on some level, responsible for it. However, she also seems to think James might one day return, so the bridge isn't entirely burned. That makes an accident the most likely.
I don't have a clear answer on exactly what kind of accident occurred. It could have been simple negligence, it could have been a more physical accident, it could have been a miscarriage with a specific cause. I doubt it was something like backing over the kid in the driveway or the baby stumbling into the pool and drowning, because Bella doesn't seem to show any signs of discomfort around any of those elements. The room is the only part of the house she seems on edge about, given it's locked all the time, so it may well be that the baby just rolled over in its sleep and suffocated or that she didn't lock the side of the crib and it fell out or something like that.
I do, however, have one niggling thought about Bella's role in the accident. The last few episodes have really emphasized the fact that she was a party girl in her college days. Beer pong champion, dancer, drinker, doer of drugs. All of those habits have gradually faded with maturity, though her wild side has never truly gone away, and most notably she still had that streak during her time with James - the library scene.
At the same time, Bella is very judgemental early on about Tremolo's constant state of intoxication, showing extreme distaste for excessive consumption and wild behavior even when she was - by her own admission - much the same at that age. She's no entirely put off alcohol or fun, obviously, but she seems much more conscious of her state of mind when around alcohol, and less forgiving to those who choose to overdo it and impair themselves.
My suspicion is that, whatever happened to the baby, it was due to Bella's wild ways. Either she gave into temptation and had some drinks while pregnant, leading to a miscarriage or a baby who died young, or she was intoxicated/hungover and unable to help the baby when it needed her. This would explain quite a few things, in my mind, while also giving the player some choice in how they respond to the eventual reveal - whether they blame her for her baby's death or not will be one of the final major choices on her route, when she finally shares this information.
I'd be curious to hear what others think of this, whether the evidence lines up or if I missed something crucial along the way. If I'm talking crazy, I'll just retreat to my brick wall theory and call it a day.
