Some of your questions are beyond me. So, I can answer you only what I know.
1) Gravity is commonly defined as the interaction between masses. Since there is no negative mass, the interaction is always attractive. However, it you define gravity as interaction among neutral objects, then there is a repulsive component due to the static interaction between charge and mass. This repulsive force is neutral since it is pro-portional to the square of a charge. The characteristics of the charge-mass interaction are still under investigation.
2) The strength of gravity is based on observation. What causes such a weak strength is not known, although there are speculations.
3) It is simply not ready yet because there are many questions remain to be answered.
4) I know very little about this and so are many others. However, a problem is that all the quantum field theories neglect gravity, although all charged particles have mass.
5) String theory is at most a remote speculation. Since it does not have physical reason for its existence nor any ob-servational support, I cannot object that it is being considered as nonsense by some physicists.