I think you make numerous very valid points, but we just don't know how Ronaldo would perform in a team that doesn't dominate every game. Real Madrid are just much bigger and better than every team in La Liga other than Atletico and Barca. They have performed so poorly this season to finish third. At Real, he is guaranteed chances because they dominate possession and play in virtually every game. At United, there are numerous games where this wouldn't apply, and to fair they've hardly dominated any matches over the last six months.
And you saw last night what Ronaldo contributes without service; ie. nothing (not that this is unusual; most strikers are like this, particularly as they get older).
Ronaldo is much more likely to move to PSG where the conditions suit him; ie. it will be a massive move, he will be the focal point of the team at the expense of everyone else, PSG batter everyone in France, he is guaranteed to score goals. If he moves to United, there is no particular reason to believe that the team would get better in terms of creating chances and progressive attacking play. He is likely to convert more chances and score more goals than Lukaku, but as he scored 28 goals this season and was arguably United's second best player after de Gea, it's hard to believe that this is what the team really needs.
In terms of contributing outside the box, or playing as a winger, those days are finished for Ronaldo. And he's not a Messi with some fabulous skillset and technical ability; he basically scores close range efforts, scores headers, or does nothing.