Vader blasted his R2 unit and would have killed Luke had Han Solo not returned in time to save him. Luke was already considered an exceptional pilot by both Obi-Wan and Biggs, his piloting skills were established several times in story before the final assault. When he destroys the Death Star it is a shot he made many times in the past having "bullseyed wamp rats in his t-16" but he is also coached the whole time by Obi-Wan 's spirit. The rest of the film is mostly Luke getting his ass kicked until someone steps in to save him (sandpeople attack, getting beat up in the cantina, bumbling into the detention center and needing Leia to get them out, etc.)
How did you come to the conclusion it was 2-3 days of training? The essential Star Wars Atlas claims "The journey to Bespin takes the Falcon weeks on its backup hyperdrive." (page 186) Even if you don't treat the book as canon, traveling from one solar system to another (Anoat to Bespin) sans hyperdrive would have to take considerably more time than a couple of days. And Luke wasn't beating Vader at all in the Empire Strikes Back, Vader toyed with him until Luke got a lucky strike in, and Vader immediately chopped his hand off afterwards.
There is also lightsaber training on Dagobah in a deleted scene (ESB DVD extras) which while cut from the film, still remained in the novelization and comic book adaptations.
Nope, Yoda said Luke wasn't a jedi until he confronts Vader for a second time.
If getting your ass kicked for the majority of 2 out of 3 films is a "Mary Sue" trait, than sure, whatever.
Fan theories don't really mean anything.