Time limit for verification of Certificate of an Employee - after 25-30 year of service Management issued notice of show cause

My Sugar Industry is the oldest and 1st Sugar Mill in Asia during 18th Century. The employees are joined as NMR, later employed as Seasonal and then as Permanent over periods. During NMR service some are submitted School Leaving Certificates. On verification after 10 to 20 years Management found forgery in some cases and asked to provide SLC, dully counter signed by DI/SI of Schools. Now after 25-30 year of service Management issued notice of show cause to terminate some 8 employees.
Let me now the time period within which the Management should verify the genuineness if in any rules.
Let me now when the Management asked to resubmit the SLC dully countersigned by School Authority, it mean the previous certificate is not acceptable? or not ?
If not accepted, then how he can take action on those non acceptable (fake) certificates ?
What should be the Role of Employees Trade Union to save the employees ?
Plz treat this most urgent before 29.02.2020 to save those 8 employees.

Dear Prasanta,
If any appointment is secured by foul means like producing fake education certificates, impersonation etc., such appointment is void ab initio. The consequential termination order, if any issued later by the employer after verification, merely amounts to a declaration that such employees were never appointed. Efflux of time does not mitigate the gravity of any fraudulent practice. After all, employment is a matter of mutual confidence.
If your management intends to show some leniency in view of the unblemished services, if any rendered by such employees and upon humanitarian considerations,, the option of their resignation and payment of terminal benefit like gratuity may be considered.