Osimhen probably doesn't want to be at Napoli anymore because Napoli don't play in the Champions League competition, I think that's a natural thing for a player like Osimhen, it's appropriate for him to go to a much stronger team.
Meanwhile, NApoli's plan to recruit Lukaku cannot be separated from Antonio Conte's wishes because Conte is quite happy with Lukaku and I think from what has happened in the past Lukaku's quality will suit Conte's coaching style quite well.
Even though Lukaku's performance has really declined at the moment, I think every coach has his own point of view and we have to respect that.
If Osimehn doesn't wants to remain at the club, I don't think he will extend his contract to 2026. I believe that Osimehn extended his contract because he is happy to stay at the club, but currently, I don't think Osimehn will remain at the club next season because his play time is limited since Conte is happy to sign Lukaku to the club, we can still view Lukaku as the best striker at the age of 31, but we cannot compare him to Osimehn at this level.
Among these candidates, I think Milan might be more keen on bringing De Zerbi. Because he is Italian and a decent manager at the same time. Milan cares about working with Italian managers more like I said before.
Other than these names Milan is rumoured to be interested in Allegri as well. You know they worked with Allegri many years ago. He managed Milan for nearly 4 years and left in 2014. He won a Serie A title with Milan too. The reason why they parted ways was the high performance drop in Milan in his fourth season. But honestly I think Allegri should be considered as an option seriously. I believe he is a very good candidate and successful at the same time.

Why should Ac Milan employ Allegri? I don't think AC Milan needs to bring back their former manager. Allegri managed for almost four years, winning the Seria A championship between 2010 and 2014. Milan wants a new manager with new strategies, they do not need to hire a manager who failed to improve this season, Juventus has the best players to win their games this season, but they struggled to win their games. Milan will compete in the next Champions League and the manager must prepare the players to play at a high level. Allegri has failed to improve Juventus, who only competed in the Seria A this season. So Allegri is not the perfect choice for Milan.