Yes, both the new and edited records will be persisted.
This would depend entirely upon the grid and really has nothing to do with StrataFrame. One thing that you could always do is cycle through the BO and reject any changes for rows that have been "Modified" but leave any rows that have been added. But this could be confusing to your end user and your would more than likely want to make existing records "read-only" within the grid and only allow new rows to be added.
The BO will handle as many new rows as you add...as for the interaction within the grid, again this depends entirely upon how you program and setup the logic within the grid.