Split: I think it would be nice to have the option to have the text document named after the first words of the beginning of the split point. Unfortunately there is no undo for split, I assume it is a major programming issue and not an oversight.
Merge: adding a contactual menu like so: select the text documents you want to merge in the binder, right click and select merge in the contactual right click menu.
Shift-Cmd-M, actually, but yes. The reason why there is no merge in the contextual menu is that you can only ever right-click on one document at a time. Merge requires at least two documents to be selected.
The former, yes. It’s a pretty significant action that involves the creation of potentially up to four new files (internally speaking). One single item in the binder is a good deal more complicated internally. Merge is consider to be the de facto Undo for accidental splitting. It’s not, in every situation however, completely seamless. Nothing gets lost, but things can change when you merge. For instance, depending upon your merge settings in the General preference pane—you could have it set to split the two merged documents with three hyphens. And some things like Document Notes can end up getting doubled (since Split retains them, and Merge then merges them). So it’s not safe to assume that Merge is a lossless action (and thus worthy of being blessed as a real Undo). It’s however safe to recommend it as a user-directed “undo”.