It’s been ages since I watched any of Gregson’s stuff – I can’t remember if the way he speaks about it implies that it’s more of a mental illness rather than trauma (something caused by a stroke or head injury, for example) or if that’s still fairly open ended.
Regardless, on the one hand, having been a caregiver, I feel like I should be sympathetic to the fact that it is INCREDIBLY overwhelming to care for someone who is not connected to reality, and that a wide variety of conditions that can be treated or supported in the community today would have been less well understood and much more difficult to manage back then, so it may make perfect sense that she’s in an institution, and he may not have planned to abandon overseeing her care after divorcing her. But, on the other hand…
I feel like I just don’t trust him as far as I can throw him because it came up in the process of him convincing Edith (or Fellowes convincing us) that it was a grand idea for her to be involved with him. So while I’m sure Fellowes genuinely wanted us to believe he was a good guy in a bad situation, I think I just spent the whole time suspicious that he was exaggerating the extent to which she was disconnected from reality. (None of that has a solid basis in canon, though, to be fair – it’s just my suspicious mind.)
I would definitely love to see more things where Edith takes an interest in her instead of it being yet another entirely dropped line. (I mean, I’m also clearly very much on the side of wanting the version where Edith realises that Michael was not giving her the full picture, so I love a universe where Edith and Lizzie could end up friends – or more than friends.)