∙ Why doesn't the negative charge on the balloon flow into the wall once the wall and balloon make contact?
Both the balloon and the wall are non conducting. It means that the electrons are bound tightly with the atoms/molecules of the material. With rubbing, enough energy is given to the atoms of the hair by the balloon to extract electrons which stick to the balloon surface due to the electric fields of the atoms , by distorting their charge distributions.
∙ Don't the excess of electrons on the balloon want to equally distribute themselves among the balloon-wall structure?
The excess electrons on the balloon due to its non conductive nature will remain locally on the surface in energy levels that become available once the electron charge distorts the atomic charge distributions. They will slowly dissipate acquiring energy from the thermal energy of the balloon. Touching the balloon to the wall will transfer some electrons depending again on energy levels activated by the new charge distributions.
On a single atom single electron level: The electron from an atom in the hair is hit by a virtual photon created by the rubbing, and then is attracted with an exchange of another photon to an empty energy level of an atom, turning it into a negative ion and releasing a real thermal photon. That is why there is heat with rubbing. This extra electron is lightly bound, and thermal energy is enough to supply a virtual photon that will free it.
In conductors dissipation is fast because there exists a collective energy level where the electrons can find an energy level niche that is common to all the conductor.