You are absolutely right, the span element isn’t “cursorable” Could you explain what do you mean by ‘non-atomic’, are those nodes with a single character in it?
“Cursor motion keys”, are you referring to arrow keys, or also click/touoch events (that sounds heavy).
Regarding marks. The reason why I had to step away from marks is because I listen for many events on that “removed content”, also showing popups next to it, so I opted in to custom NodeView. It was all possible with mark/markView. But there was one specific reason that prevented me from using marks which I haven’t reflected in my apps code 
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Although I thought that the node was the right choice because it represents the block of text as a united entity
Could you suggest if I try working with backspace/enter key bindings would be the right direction?