Delete
On the title bar of the application window, you can click Undo to undo the last action.
When you delete a task that was added from a source wireframe, the task is also deleted from the cached layer.
One way to quickly select multiple tasks is to right-click on a task and select the Select | All Linked Tasks option from the menu. All of the (predecessor and dependent) tasks that are linked to the current task are selected.
Critical Path
To select all of the tasks on the Critical Path of a group of linked tasks, right-click on a task and select the Select | Critical Path option.