1) un regsitered leave and licence agreement would be valid
2) if document is under stamped you would have to pay deficent stamp duty with penalty
3) you should not go to court . you are in possession of the property . let landlord file case in court for your eviction
4) you should in reply to landlord notice mention that contract does not provide that tenant is liable to pay maintenance
5) mention the money invested t by you on upgradation of the office premises