BTW , do you think it is reasonable that Italy will still fight after Germany surrender ?( although Allies already has the advantage )
You mean, IRL? They would probably sign an armistice when Germany surrendered, as their situation would become hopeless and Italian morale during the war was never as high as German morale, nor was the Italian leadership as fanatical as the German one. However, for game purposes, changing this would be a bad idea, since Italy could be effectively ignored and the game would get easier for the Allies, considering the in-game VCs. One might wonder whether the western Allies would sign an armistice with Germany after the potential fall of the Soviet Union, too, but considering the VCs and game balance, any changes in that area would be bad for the game. CEAW's politics is static and created with semi-historical gameplay in mind. Changing this without changing the whole game structure is probably impossible.
Moreover, as far as ahistorical outcomes are concerned, we can give the game the benefit of the doubt and just pick what's best for gameplay. Changing history for the sake of gameplay is one thing, but you cannot really change sth which never happened. We can make an educated guess at best in case of alternative history, anyway.