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.