Primary Aldosteronism (Conn's Syndrome): This condition is characterized by excessive production of aldosterone, leading to hypertension, hypokalemia, and sometimes metabolic alkalosis. The low plasma renin activity and normal serum aldosterone in the context of hypokalemia might seem contradictory, but aldosterone levels can be normal in some cases due to the body's attempt to regulate the imbalance. The presence of hypomagnesemia and hypophosphatemia further supports this diagnosis, as aldosterone can lead to increased excretion of these ions.

Liddle's Syndrome: A rare genetic disorder leading to excessive sodium absorption and potassium secretion in the kidneys, resulting in hypertension, hypokalemia, and low renin and aldosterone levels. The normal serum aldosterone and low plasma renin activity, along with the patient's symptoms, make this a plausible diagnosis.
Gitelman Syndrome: An inherited disorder affecting the kidneys, leading to hypokalemia, hypomagnesemia, and hypocalciuria. While it typically presents with normal to low blood pressure, the presence of diabetes mellitus and hypertension could complicate the clinical picture.
Bartter Syndrome: Similar to Gitelman Syndrome, it involves defects in renal ion transport, leading to hypokalemia, hypomagnesemia, and usually normal or low blood pressure. However, the hypertension in this case might be secondary to another condition.

Cushing's Syndrome: Although the serum cortisol is reported as low, this could be due to the timing of the sample or other factors. Cushing's Syndrome can cause hypertension, hypokalemia, and glucose intolerance (potentially worsening diabetes mellitus). It's crucial to consider this diagnosis due to its significant implications on morbidity and mortality if left untreated.
Pheochromocytoma: A catecholamine-secreting tumor that can cause episodic or sustained hypertension. While less directly linked to the electrolyte imbalances, it's a critical diagnosis not to miss due to its potential for severe cardiovascular complications.

Familial Hypokalemic Periodic Paralysis: A genetic disorder leading to episodes of muscle weakness or paralysis, often associated with hypokalemia. The persistent hypokalemia and presence of loose motions could be related to this condition, although it's less likely given the other symptoms.
17-alpha Hydroxylase Deficiency: A rare genetic disorder affecting steroid synthesis, leading to hypertension, hypokalemia, and low levels of sex hormones. The normal serum aldosterone might not directly support this diagnosis, but the complex interplay of steroid hormones could result in unexpected laboratory findings.