A 58-year-old woman presents to an outpatient clinic complaining of persistent and worsening stiffness in her left shoulder over the past six months. She describes significant difficulty with everyday tasks such as fastening her bra, reaching into overhead cabinets, and sleeping comfortably on her side. She initially experienced a dull ache, which has now progressed to constant stiffness and sharp pain with any attempted movement beyond a limited range. Her medical history is notable for long-standing type 2 diabetes mellitus, obesity, and well-controlled hypertension. Current medications include metformin, insulin glargine, and valsartan. She reports a largely sedentary lifestyle due to her shoulder pain and general fatigue. Her vital signs today are: temperature 37.1°C, blood pressure 132/80 mmHg, pulse 70/min, and respirations 14/min. On physical examination of the left shoulder, there is marked global limitation of both active and passive range of motion in all planes, particularly external rotation and abduction. Deltoid and rotator cuff strength testing is difficult to assess due to severe pain and restricted movement, but gross motor function appears preserved, limited by pain to 4/5. No crepitus, warmth, or erythema is noted. Based on these findings, what is the most appropriate initial management strategy and potential long-term complication if conservative measures fail?