Dear Ashu Ori,
The S.O 2806 (E) dt 28-08-2017 is the notification issued by the Central Govt enhancing the coverage limit of wages u/s 1(6) of the Payment of Wages Act,1936 from the existing Rs.18000/= to Rs 24000/= per month for the purpose of application of the Act. Sorry, therefore, you should have most probably misquoted the learned advocates.
Terminal benefit, as the very name would clearly suggest, is the benefit payable to an exiting employee by the employer on the occasion of termination of his employment under him. Logically, therefore, what the salary/wages the employee has drawn lastly at the time of termination of his employment is the monetary basis and the total no of years of continuous service rendered by him is the chronological unit for computing the benefit and not any average as you have mentioned.
Regarding your query about the employment status of accountant and supervisor, I request you to go through the definition clause u/s 2(s) of the ID Act,1947 once again. Then you will agree that irrespective of the salary drawn, an accountant is a 'WORKMAN" and a supervisor can also be so when his monthly salary does not exceed Rs.10000/=.