Quite right, all of these factors and more are the sum of the whole in the Peninsula.

In 1812, the point is that while Alexander showed greater strategic sense by ensuring that he would fight a one front war, Napoleon did not. So when Napoleon lost heavily in Russia he did not have enough trained veteran men readily available as a strategic reserve, in Central Europe, on which to quickly rebuild his army. As Russia also lost heavily, those veterans of Spain could have made the difference in early 1813.

However, by 1812 I think it unlikely that Spain would have accepted a peace with France any more than they did in early 1814.

Still, those 100,00 men in the south of France could have made the difference serving under Napoleon in 1814. Two of his best Marshals and excellent generals were fighting in the south.