I think you overrate the PL, when Messi and Ronaldo were both in La Liga it was the strongest league in the world. You only have to see the points totals Liverpool and Man City have got since becoming top teams to see that and neither have players that are of Messi's calibre.
Because Messi was the best player on the pitch over the two legs - he scored two goals and created about 6 goalscoring chances. You picked a terrible example.
Messi created space by taking it past 2 players with 4 touches within a second. There's 7+ players ahead of Messi when he receives the ball and he takes it past half of them by dribbling.
Plenty of teams allow that space, the premier league isn't as unique as you think it is. If Messi can still score 2 goals and create various chances vs. Liverpool, a team that suffocates the space more than anyone else in the PL (or club football, probably), imagine what he'd do against Huddersfield or Bournemouth?
Bit rich to say this when your whole argument is based on the assumption that Messi wouldn't be as good away from Barcelona or in a different league. We can only go on what they actually have shown in their careers. Also Messi has played under different systems and managers - none of whom except Pep have gone on to have success after managing Barcelona. Whereas Ronaldo has been coached by all-time greats and top managers like SAF, Ancelotti, Mourinho, Zidane, Allegri.