I personally believe, this problem is because Conte does not want Osimhen and then, Osimhen is available for sale but at the same time Napoli still want Osimhen to stay. Because anyway, with the Napoli factor extending Osimhen's contract last season is also at least proof that Napoli want Osimhen to stay at Napoli longer. And also, last season also at the time of the contract extension, that Osimhen did prefer to stay at Napoli rather than leave.
You are right, with the look of things I think Conte is the reason behind everything, however considering how new he is in the club he shouldn't make some kind of decision especially the ones that involves selling a player because he has not really work with the club to be certain of how the players perform, so for me I think what he should have done is to give Osimhen a chance to prove himself instead of that sudden dislike, however the negative aspect of it is that since Conte has a wrong perspective about Osimhen it may likely affect the performance of Osimhen, though Lukaku is not a bad idea but since Osimhen still love to play with Napoli I think he should be allowed to stay more longer because he is still an assets for the club.
I do agree that Conte has a lot of say in the club now, because while Napoli is still good and all and yes they won a title once with some luck, I still think that Conte has a bigger name than the entire Napoli club history.
This is why I think we should consider that Conte having some say in it, sometimes some coaches go to much smaller clubs than they should, only to gain some power. They ask for total control, because if Conte was managing a team like lets say City or Madrid, he would be judged, and he would be in limited power as well, but when he is at Napoli, that means he can do anything, including even selling or buying players as well. Insisting on Lukaku is one of those situations, because he knows that he can use him very well and he knows that Lukaku played well at Serie A before and he can bring Lukaku back to his prime as well.
This is why they are trying so hard to get as much from Osimhen as they can, basically they are asking one Lukaku amount more than they should, so when they buy Lukaku, what's left would be good enough money for them. The best thing is that we are talking about something that will take some time, and I am not really seeing how Osimhen could be sold for that much but it would work great for Conte if they can sell him.