G is called free energy because this is the amount of energy that is free to do work. The products of a reaction still have energy in the form of bonds, but that energy is not available to do work. A reaction is spontaneous if G is negative, that is, the products have a lower Gibbs free energy than the reactants. Note that depending on the sign of H and S, the spontaneity of a reaction can be temperature dependent. Don't confuse this temperature dependence with the kinetics of a reaction, which increases with higher temeperature because it is easier to get over the activation barrier.
Cold packs get cold when a salt mixes with water.
No, the probability of that arrangement is very very small. The more likely arrangement is 90 people more or less randomly distributed in the room. This arrangement is more probable and we could quantitate this more probable arrangement in terms of entropy.