The change in any variable of state of the system during a process depend only on the initial and final states of the system, not on the paths taken. Change in variables such as q or w DOES depend on the path. For a cyclic path, there is no change in any variable of state . The procedure is to calculate the energy change and enthalpy change from the initial to final states by many reversible steps. We can calculate the D H for other T as we did previously, using D Cp for all products and reactants multiplied by their stoich.
Resonance energy stabilizes bonds with electron delocalization.
More complex molecules have more ways to store energy before T rises and hence higher heat capacities. For real gases, liquids and solids, molecular interactions can also store energy, resulting in higher heat capacities.