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The British weren't going to invade the Balkans. The Germans were tied up just having to send troops there in case they did invade the Balkans and the British were happy to leave them there. The British only set foot in the Greece(well other than those send in secret already) when the Germans were already leaving and even then it was to secure the region and try to prevent a civil war between the Communists and others, once the British army did get involved in that Alanbrooke became quite frustrated as it was now tying up British forces for little purpose.
Going for Genoa would have required taking Corsica and Sardinia first and then it places them too close to France. Part of the reason for the invasion was to bring German forces away from France in the first place. I think it would have been easier of Germany to throw off an invasion there as well. Having Taken Sardinia and then Corsca the Germans would have an idea where they were headed next and would not just have the forces in Italy to deal with it but there would be the forces that had been defending southern France too. Plus it is easier to send troops to that area than to Southern Italy, it's closer and would be less vulnerable to allied air attack.