I got most of the line editor working and adding in the functionality to explode multivalues to make it easier to edit was straightforward. I'm happy with how much the editor actually works.
By ignoring most of the complexity of an editor such as undo history or working about memory, you can get something quickly working.
I got most of the functionality I want working, I still need an undo function but for now I think I'm happy with the state of things. If I used the editor I would consider it not a toy and actively work on it but I'll only be using it to edit multivalue sets in D3 so for now this is just a toy.
Definitely recommended working on a line editor!