Temporomandibular Joint (TMJ) Disorder: Given the patient's symptoms of pain radiating from the left submandibular area into the jaw and sometimes into the ear, especially with eating, TMJ disorder is a plausible diagnosis. The dull ache nature of the pain and its exacerbation with jaw movements (like eating) align with TMJ pathology.

Cervical Radiculopathy: The patient's history of cervical disc disease, particularly at C5-C6, could explain the radiating pain if the nerve roots involved are irritated or compressed. The pain pattern might not perfectly align with typical radiculopathy, but the cervical spine's involvement cannot be overlooked.
Dental Issues: Toothache or dental problems could cause pain that radiates to the ear and jaw. Although the patient mentions a dull ache, dental issues should be considered, especially if the pain worsens with eating.
Myofascial Pain Syndrome: This condition involves pain and inflammation of the muscles and their surrounding soft tissues. Given the patient's symptoms and the location of the pain, myofascial pain syndrome, particularly affecting the muscles of mastication, is a possible diagnosis.

Lung Cancer: As a former smoker, the patient is at higher risk for lung cancer. Although the primary symptom described is localized pain, it's crucial to consider that lung tumors, especially those located in the apex (Pancoast tumors), can cause referred pain to the face and jaw through involvement of the cervical nerves.
Giant Cell Arteritis: This condition, although more common in older adults, can cause jaw claudication (pain when eating) and should be considered, especially if there are other systemic symptoms like fever, weight loss, or visual disturbances.
Trigeminal Neuralgia: Characterized by sudden, severe, shock-like pain around the eyes, lips, or nose mainly because of abnormal blood vessels pressing on the trigeminal nerve in the head. Although the patient's pain is described as a dull ache, atypical presentations can occur.

Eagle Syndrome: Characterized by an elongated styloid process or calcified stylohyoid ligament, which can cause facial, jaw, and ear pain due to irritation of the surrounding nerves. This condition is less common but should be considered if other diagnoses are ruled out.
Osteonecrosis of the Jaw: More commonly associated with bisphosphonate use or radiation therapy, this condition could cause jaw pain. Given the patient's history of smoking, vascular issues might contribute to a rare case of osteonecrosis.
Neuralgic Amyotrophy: An uncommon condition that affects the nerves going to the shoulder and arm, but in rare cases, could cause pain in the jaw or face if the nerves involved have a broader distribution or if there's an atypical presentation.