Don't worry, despite having worked for such a long period, especially in a senior position, you should be more than capable of advising others. Nonetheless, if they do not consider issuing an Experience Letter, at least you are entitled to receive a 'Service Certificate'. This certificate format does not include detailed performance appraisals; it simply comprises the Name of the employee, Date of Birth, Date of Joining, Date of leaving, Designation upon leaving, Emoluments drawn upon leaving, and lastly, Remarks. This type of certificate should fulfill your purpose. Request this, and if it is not provided, take legal action, issue a legal notice, and claim damages for the harassment you have experienced. As a last resort, this may expedite the issuance of your Experience/Service Certificate.
Based on your description, absence without prior leave sanction does not warrant a severe punishment like 'termination'. Primarily, you should challenge this termination itself by demanding reinstatement with back wages and continuity of service. Perhaps, taking such a step might accelerate the process of obtaining your Experience/Service Certificate. In my view, if 'Absence without leave sanction' were considered a grave offense, they would have initially forfeited the gratuity itself, so I do not believe they have a strong case to defend against your claims. Remember the famous adage, "offense is the best form of defense." Be courageous in facing any outcome, as long as your competence is not in question. I am confident that speaking the truth will lead you to a job of your choice. All the best, pal.