I don't know what plans Chelsea have for replacing Lukaku but Christiano Ronaldo moving to Chelsea seems very unlikely. He wouldn't do that to Manchester United. Lewandoski might be open to a move to Chelsea and with his price tag Chelsea are in a better position to get him than Barcelona at the moment and I think he will be open to the idea of moving to the English premier league.
You are right, if Ronaldo moved it would probably be to another league, we saw at the beginning of his move to Manchester United, that the first one closer to this Portuguese star was Manchester City, even the news has spread so widely. But those behind Ronaldo didn't let that happen and did something so he joined Manchester United again. I think it will happen again if another team in the Premier League wants Ronaldo.
Yeah I actually agree with you, I don't see Ronaldo playing in Chelsea or any other EPL teams, he is a player that has high potential and love for Manchester United, so moving to their rivals club which is Chelsea won't be possible, if ask, instead of opening talk with Ronaldo, the blues should either go for lewandowski. Lewandowski is recently opened for any top European clubs, though barca are seriously considering bringing him to the club, but with the recent financial challenges they are having, I don't think they can bring in lewandowski to the club due to his price tags. So Chelsea might be lucky if they go for him, cause what is keeping him at Bayern Munich is his price and he also need a club that can Meetup with his salary.