The field [Total Slack] show the difference between Late Finish and Early Finish or between Late Start and Early Start (whichever is smaller).

I've re-created your project file using Project 2010 so let's step through this.

Before resource leveling all tasks are critical (zero total slack). This makes sense because all of the tasks assigned to Resource B (Tasks A - K) all have successors and all tasks have the same durations.

After leveling the only two tasks that are critical are tasks K (last in the tasks assigned to Resource B) and task X.

Because none of your tasks are linked past the simple one link the only tasks showing as critical are the last. You are correct that if Task A slips, it will delay task N - but Task N is not the last task in the project. If you want to see that A will Delay N (a single network path) you need to turn on the option View Multiple Critical paths.