The design force on the balcony equals the dead load of everything above it (construction material, fixed objects), plus the live load of everything above it (people, movable furniture), plus the dynamic load due to wind, earthquake, impact. You are only going to increase the dead load by putting decking material above the concrete, you will NOT decrease the total load on the deck, and unless the material you place is designed to carry structural load, you will not increase the strength of the deck.
If the construction is as bad as you suggest, you should consider demolition of the deck before someone gets hurt.