I don’t see Christiano Ronaldo switching sides to Chelsea. He’s a responsible player and knows what rivalry means. I am aware that Chelsea Owner met with Ronaldo's Agent to discuss about a possible transfer but it is not reliable. I think people might have thought that his frustration over Manchester United’s lack of activity in the transfer market while other top-six Premier League clubs have continued to bolster their squads is the reason why he wants to leave. Ronaldo is an important part of Erik Ten Hag's plan so there’s no point if he leaves. Chelsea will have to look elsewhere for a new striker to replace Lukaku as Lewandoski has only Barcelona on his mind at the moment.
If Ronaldo ends up moving away from Manchester United during the summer transfer window, then it will only make Ten Hag's job even more complicated. I think you're right, Ronaldo was included in Ten Hag's plans this season because his quality and experience can still be relied on in attack for Manchester United. Although I can think Ronaldo is just pushing the Manchester United management to be more active in the market, but everything is still possible because other teams still have a chance to convince Ronaldo to move.
Of course it would be very detrimental to Manchester United if Ronaldo moved to another team in the transfer market this summer. So the management of Manchester United must try to be more active in the transfer market if they want Ronaldo to stay in the team. Regarding De Jong, maybe this transfer will not happen as long as Barcelona are still reluctant to accept Manchester United's offer. So they have to look for other alternatives to strengthen their team before the league starts.