Yes it is all speculation as to the turn of events if Napoleon had withdrawn substantial forces from Spain in early 1813.

Historically, Wellington delayed crossing the Pyrenees into France until the situation in Central Europe became clear. That is no separate peace which would have allowed Napoleon to take the offensive against Wellington in southern France.

The Allies cleared most of Spain to the Pyrenees by the end of June/July. If Napoleon had withdrawn substantial forces from Spain in early 1813, the Allies may have been on the frontier months earlier.

While the situation was still uncertain Wellington was willing to "bend to the views of the allies if it can be done to the safety of the army..." 19 September 1813. Although he preferred to clear Catalonia, he crossed the Bidassoa 7 October 1813. Leipzig was 16-19 October 1813. He issued stringent orders for the conduct of his army and enforced it to ensure that the population remained at least neutral. In this he was proven correct.

But yes much would depend upon what Napoleon did with those veteran troops now in Central Europe and the reaction of the Allies there.

Good hunting

Ron