It seems act of threatening and not ethical from your ex-employer side.
However you should be very sure that you have cleared all dues payable to your employer related to your wife business. Hope you have proof for the same. Show cause for indulging in other business during the course of employment, even if it is genuine, is not valid when you were already from the services of the Company. The employer- employee bond is no more after relieving.
Seek the help of Labour Advocate and reply to their notice stating that you have been already relieved from their services of the company and all allegations are false.
Send them a separate letter to clear settlement and gratuity and keep a copy of the letter. In case of failure from your employer end you can take it up further with Labour department.
Laws may be invoked against anyone such as yourself only when there has occured any Unlawful act actibvity or abetment thereto
So long as there is no scope for the previous employer for filing an FIR against you the show cause notice can be ignored completely. Then regarding gratuity, you can file a case before the Appropriate authority, should be the Labour Officer of your area.The employer cannot deny it saying that you were charged for misconduct because in order to attract section 4(6) of the Gratuity Act which tells about forfeiture of gratuity, the employee should have been dismissed from service after conducting an enquiry for a charge of gross misconduct. charging less price is a misconduct and if the company has incurred any loss due to that he can recover it from your gratuity. But it is not that he can relieve you and then frame a charge against you and then forfeit the gratuity. Therefore, having been relieved from service the employer cannot initiate any disciplinary action against you.
You have resigned your job and your resignation has been accepted and you are also relieved vide company's release letter. Therefore the employer employee relationship has effectively come to an end from the date of release.
Any show cause notice after this date is not maintainable . You go on record in reply that you deny all the allegations as the same are false and motivated.
Thereafter don't reply to any subsequent correspondence and move on. Company's any action in this regard is not sustainable in any labour court.