1. If the second suit is filed on the same cause of action with the same prayer then you may file an application for res-judicata.
2. If suit property already suit prior to the grant of stay under Order 30 Rule 1 and 2, then the stay has no value. If there is a order passed before selling the property, then the said Order will continue in perpetuity. It has to be permanent injunction Order passed in the Suit. There is no limitation.
3. No they cannot be added in the application if they are not the party in the suit.