There was no German spy network in England from about mid 1940 on. The few agents still in the field were all double agents working for the English. The rest were in prison or had been hug. If you read on the German efforts to put agents in England they read like a Three Stooges comedy. Guys that couldn't speak English let alone without an accent, had no clue where they were, given nothing to accomplish their mission worthwhile and, just plain clueless. The only intelligence the Germans were getting was through their Y service (radio intercepts). This gave them the bare bones basics of what was going on nothing more. By 1943 even aerial photography was getting sketchy.
If the Allies continued to demonstrate that they were more likely to move against Italy in the months preceding the invasion they could have given enough cover to their actual intentions to avoid a German buildup.
Besides, Hitler and the OKW were occupied with Kursk and the offensive there.

As to the invasion, I gave the initial wave as roughly the same size as the original D-Day. The Allies would have then brought over additional divisions just as they originally did. The big bottleneck in doing a 1943 invasion hinges on one item: LSTs. By depriving MacAuthur of his allotment and using those assigned to the Med there would have been sufficent for a 1943 landing.

The biggest Allied advantage is that the Germans had just two panzer divisions in France along with components of several others at the time. There were fewer divisions on the Atlantic Wall and most of these were already in poor shape manpower-wise. The typical infantry division was static, had older draftees, men invalided for frontline service (eg., wounded or other medical problems but still fit enough to serve in light duty) as well as the usual dregs. Most had been culled several times for replacements. Few had even trained as full divisions and most were occupied more with building fortifications than training for combat.

With Kursk occupying German attention this would have given the Allies time to get ashore and established before the Germans could mount a counterattack. It would also cripple their attack in the East by forcing the withdrawal of some mobile forces to contain the Allies in the West.