So he disappears/goes mia or whatever and then she suddenly starts hallucinating the guy when he's not really there? That isn't any better imo.
Have you seen Fight Club? How the narrator of the story spoke of Tyler, then at the end we get a reveal of what's actually happening? Or Inception for that matter, when Cobb is seeing his wife, who we know is dead, but Cobb isn't sure if he's dreaming or is actually awake. Same thing can be applied to Bella, who can't get over the death of her husband and if James is really dead, then you can even connect it to the locked room - it being his study, home office or whatever.
I understand what you mean. But I think for this flashback to give us something different. James mentions something he and Isabella once did on a beach. Oddly enough, after EP8 we get the interlude where it's about surfing. So sea and beach. In my opinion, DPC shows us with the Bella flashback, the beginning of the end of the relationship of Isabela and James.
That may be possible. We know Bella hates adulterers, so it's implied that James cheated on her. So if this flashback is about her starting to reject him and he starts looking for it somewhere else, what made him leave/disappear? Besides in that pool scene with MC, Bella's monologue states that she hopes James' forgave her as well, what would that be for?