Many false/incorrect assumptions you're making there. First off Messi's injury problems were fixed by a combination of dietary changes as well as exercise regimen and a daily plan. He has a medical trainer with him at all times (even during Argentina games) and has an extensive stretching and warmup session 30 minutes before and after training. He (like all of the players) takes multivitamins and various other nutritional supplements to promote muscular endurance and healthy blood flow. He also has a necessary daily and sleeping schedule that he follows religiously. You can write off all these medicinal and physical regimens/diets all you want but the fact is that athletes need them to stay fit and they are among the most important aspects of the game. The most disciplined athletes with the most detailed and helpful systems are often the ones with the least injuries.
Messi's distance coverage is lower because of conserving energy in my opinion. It seems stupid but he does play up to 70 games in a season and he can't afford to run like a dog every single game otherwise he risks burning out. Also, he saves that energy for in game situations. It's quite clever actually. What's the point of running around crazily trying to set up an attack when the opposition defenders are watching you like hawks and closely marking you, while they have high levels of energy, and when there are barely any openings from which to set up a decent scoring chance. He saves his energy for when he sees them divert their attention elsewhere momentarily or when they are too tired to keep up with his trademark bursts of acceleration or when he sees a gap in space and can create a chance for his teammates. It's a smart thing to do and shows that he is always 3-4 steps ahead of the opposition. He can't just be lucky enough to score 70 goals in a season. He has the intelligence and instinct to always be making those opportunities.
Lastly, I don't know where you got that point where you said that he's closer to the opposition goal. Messi plays much deeper than he ever has with each coming season and rarely stays glued to the opponents defensive line waiting to make a run. He is the center of the mass of attacking players and as they pass around and link up they all move forward with Messi still in the middle. Then as the team is surrounding the opposition box it only takes a few seconds of finding a chance and exploiting it for Messi to make his move and play a one two or anything else for a goal to be scored.
My bad, I should have added the word 'just' after 'not' in "Not because of oil fish or diet.
You attributed the low distance coverage due to energy conserving, And avoiding long term fatigue, Yet somehow forgot to add the injury risk, Which was my exact point, Players who exert themselves over many games have a higher risk of injury.
Messi doesn't play deeper than last season, It's the opposite, , When he used to help in 'pressuring' in previous seasons, He ended up recovering the ball deeper in the field, With his defensive duties gone now, He can stick around farther up the pitch and receives the ball the ball there, Making him more dangerous, This lead to the reoccurring theme this season of messi just 'walking' high up while his teams mates are fighting for the ball, Some members were puzzled by this and ended up criticizing him.
This holds true for most games, But in CL games it can be different since opposition doesn't make it easy for the others to pass him the ball, And he gets forced to drop deep to receive it, Of course this is just one of multiple advantages of dropping his defensive duties, It can also help keeping him more fresh within a match, And concentrate his energy on his runs and finishing.