So instead of a nursery or some room dedicated to James, it's more like the scene of the end of their marriage? As for the parents.... hmmm, maybe it used to be Bella's room as a child? We've seen pretty much her entire house, there's only one bedroom. This could be a second one, BUT that would be a pretty lame reveal.
With the shocked look on Maya's and the MC's faces when the door was opened at the end of the episode it can't be just a normal looking bedroom.
Whatever they saw was totally unexpected.
(I get narratively why DPC had the door be unlocked, it led to the episode dramatic finish. But it's pretty unlikely that Bella kept the door locked previously but the night she has 6 guests over for dinner she forgets to lock it.)
I think by far the best theory is the miscarriage and nursery. Maya and the MC saw a crib and nursery items.
It's possible that it wasn't a miscarriage, maybe the child was born but died as an infant. SIDS or something.
The dead child was a girl as well.
Remember when the MC joined Jill and Bella in the park when they were doing yoga early in the game?
After Jill left with Tybalt the MC was arguing with Bella because Bella was being very protective of Jill.
The MC said "she's not your daughter" and then Bella had this brief but sad look on her face, just a second or so.