Would a CBC always show leukemia, if you have leukemia?

Usually. The cbc, or complete blood count measures the normal cellular components of blood, red cells, white cells and platelets. Leukemia is detected by either there being abnormal cells in the blood, or a reduction in the normal elements. Very early on in the development of leukemia, there might not be enough damage to the bone marrow for the normal elements to be effected or abnormal cells to be found.
Not always. Yes, in acute leukemia - you could see immature cancerous cells- called- blasts on the WBC differentials/ peripheral blood smears. In chronic leukemia however- you could not see the cancer cells you could see some abnormal changes there in CBC and these would raise your suspicion towards possibility of chronic leukemia- (cll/cml) however other work-up would need to be done to establish a diagnosis.
No. The CBC only shows the total number of white blood cells, red blood cells and platelets. In order to diagnose leukemia, one must look under the microscope at many characteristics of the blood cells. Usually this requires a test called a bone marrow biopsy.