I like this feature, too. But I'm a little uncertain about some of your examples so perhaps I can ask here.
If you're editing a numeric matrix (or a numeric cell in a het matrix), you can just insert/delete/overwrite digits in a multi-digit number just like you'd expect from any text editor. But if you're editing, for instance, a 2-D matrix of nested text strings, how do they appear in the cells and can you edit them just by typing in the same way you'd edit a multi-digit number, or do you have to open up a "nested element" editing window to change it?