Lukaku is just ap layer they know they are getting what they are buying for and they are not going to be shocked with whatever happens. That's sometimes good enough, yes it is not a marvellous trade or anything like that but it is at least a known trade and we should probably consider that this could be a good investment for them. I know that it is going to take a while for him to be getting back into match shape but he is still a good player, and good enough for Napoli level as well.
You need to remember that not every team tries to win the league and while Napoli win it in recent years and would love to win it again, they are aware that they are not going to get it again. In that case, Lukaku isn't a bad player for a team that is pushing for fourth place at the best case in their mind.
I don't think so, it's clear that Lukaku's performance has declined significantly in the last few seasons and there are many factors that cause this, including Lukaku's age which is no longer young, so I think he will not be able to perform optimally again like his former glory.
Apart from that, Lukaku is a player that Conte really wants, we don't know for sure why but every coach has his own criteria and perspectives to be able to carry out the tactics he applies.
Napoli are now in a slump, we can't compare them to a few seasons ago when they won the Scudetto and for a team that is already in a slump it will not be easy to rise, now Napoli must start from the beginning again in building this team and that will take a long time.