Approach the matter methodically.
It can't be a miscarriage because there are these children's stairs in Isabella's bathroom. Ergo there was a child. The only question is whether the child who once lived in Isabella's house was her own child or not. You should also ask yourself whether this child is still alive or not.
I think to myself so that it wasn't Isabella's own child and that child was still alive. Isabella herself implies in EP8 when she cut the MC's hair that she knows about cutting hair. So it's not that unlikely that Isabella always cut this child's hair. Practice makes perfect.
Because I'm mean and want to have some fun, you'll get a screenshot from me.
As for me, that second mirror and "children stairs" looks like an accessory for intimate hygiene (shaving etc).
Such stairs are not necessary for a children older than 7-8 years and are dangerous for younger ones. What is the point of it at all, given the height at which the mirror hangs in front of it? What you suppose baby would do there?
The sink is to the side and much higher than the "child" mirror. I have never seen anything like this intended for use by children either in apartments of multi-storey buildings or in private houses. A ladder for the child to reach the "adult" sink? But then why that second mirror below and the ladder standing in front of it?
Adding here those lightened renders from her closed room, where MC trying to figure out what is inside, we clearly see that it's not a "baby" room. It's just ridiculous and strange to put child (baby) room in such place. It's near the bathroom which produces a lot of noise, and I doubt that walls is that capital. As I know, such buildings in USA are pretty "flimsy", walls, at least internal, usually made of light blocks, drywall, wood and other similar materials. Show me an ordinary house in the USA as in the "Bella quarter", which is built entirely of brick, or at least of concrete blocks, including the interior walls. Btw this was mentioned once is regards of DIKs mansion, when MC sparring with Sage.
This is nonsense and a rarity. At least in the southern regions and where temperatures are moderate in winter. We, in turn, know that the BADIK takes place somewhere in the southern part of the USA, because Zoey was traveling to San Diego (which in turn is close to the border with Mexico) by train and for an quite short time, hardly more than half a day. Long-distance trains look different, and not like a subway ones. Long range passengers have to rest, sleep, etc.
Child story in overall doesn't fit well in the BADIK universe. In case of child death it's changing game mood and setting to the very dark tones. Bella's words about forgiving James and her hopes that he also forgiven her doesn't give me the sense that it's about and because of their child or the consequences of his death or something like that.
We used to get the flashbacks in the beginning of each new episode. In EP9 we got a video instead of classic bunch of renders. That was a Bella's house, at the Christmas or New Year time. Probably about 3 or more years ago, when those "accident" happened. Was she a victim of robbery and rape? Was it James who made all that mess in her house that night? If so, why he did that? Who was the man walking out of that locked room? James, who was a military man and died in some kind of military conflict, is also a "normal" version. Or he was not a military man, but for example made expeditions to some exotic countries and African tribes. Some accessories and trophies we saw gives us a hint.