1) under himachal pradesh premature retirment rules 1976 you can take premature reitrement only after completeing 20 years of service and by giving 3 months notice and it has to be accepted by appropriate authority
the amount of pension is calculated based on the total qualifying service and the average emoluments drawn during the last ten months by a Government Servant.
50% of average emoluments is regarded as Full Pension of a retiring Government servant, if he has rendered not less than 33 years of Qualifying service
If the qualifying services is less than 33 years, pension will be calculated proportionately.
If the service rendered is less than ten years, no pension is payable. Only service Gratuity in lump sum in lieu of pension is admissible.
2) you can apply for NOC for joining private college in mumbai . it would not weaken your case for retirement benefits
3) you should rejoin in service and then take reisgn
4) you would be eligible for experience certifcate