I should have been more clear, sorry. After you add the empty bars, don’t use insert mode, since it will indeed move everything for the remainer of the flow. Instead, make a selection that starts where where you want to add music and ends at he new empty bars, and move that selection towards the newly created empty bars using alt-right arrow. Then enter in the new music (again, NOT using insert mode). That’s how I would do it.
It would be nice if there were a way to specify a range of bars where insert mode would apply, but I don’t believe there is.