Big advantage in my opinion. Not only they will be fresh but also they will have more time to prepare and improve whatever they need to improve during this break even if I think they are solid in all areas. I don't think Napoli will cease any of those players until next summer and if they will win Serie A , I think they will sell none of them until they have the UCL run after winning Serie A. However, I'm sure a lot of important and respectable clubs are already watching they "brains" of Napoli from the pitch. 
Sure, it is a big advantage. During the league break, Napoli players can have enough time to prepare anything for the next matches. The coach can order the players to have more time for training. This is something that the players won't get if they join the world cup matches.If the Napoli management and the coach are very serious to win Serie A title or UCL title, they surely won't sell any of their key players. They must refuse any offer in January although there is an interesting offer of big money from top clubs. I am sure there will be many offers from top clubs in the next transfer window, especially for Victor Osimhen. It is a big challenge for Napoli to ensure all players to stay until the end of the season.
If they do decide to let go of Victor Osimhen, I think it is going to be detrimental for them. They have a good thing going for them right now. And I think if they let go of the player who is scoring the most amount of goals for them, it is indeed going to be very bad for the club. He is a big reason why Napoli has a good advantage in the first place. There will be a lot of offers that are going to be finger-licking good. But they better not fall for those.
I think the best thing is going to be keeping the place that they already have and bringing in a few players that they feel like they will need going forward in the season.