With the recent changes at Napoli, it'll take a miracle for the them replicate their great performance of last season. The new manager and the new players would certainly have to work together for awhile to better understand themselves and find the best style of play that'll work for the team.
Defending a league title is considered to be harder than winning because other clubs in the league will be very much wary of the defending club.
I don't think so because Napoli only changed coach, while their squad depth is still quite strong because none of the star players have gone to other teams in this transfer window. Indeed under their new coach Napoli will definitely have a different style of play next season. However, Napoli is still very much counted on next season if Osimhen does not leave Napoli in this transfer window.
Apart from that there was no recruitment of quality players carried out by the management of Napoli in this transfer window. Development for now I see that Osimhen has not signed a new contract which could trigger a reaction to his future with Napoli in the next season. I think Manchester United is still monitoring the developments that are happening with Osimhen in Napoli in the new contract offer offered by Napoli but Osimhen doesn't want to sign a new contract yet.