The solution to Coulson's return possibly lies in a couple of the Marvel films and comics.
Firstly when you see Thor leave Earth with Loki the Tesseract is in a container that doesn’t look like it ever could have originated from Earth. Its of Asgardian design. And he didn’t seem to have it when we first saw him in “The Avengers.” So either he returned to Asgard after to get it, Odin sent it on later or another Asgardian could have returned by Odin to Earth (remember it seems to be a two person activated device)to deliver it (and for who that could have been read at the end).
What you don’t see at all in the film was any of the bargaining between Fury and Thor for the return of the Tesseract. I’ll wager the reamination of Coulson was part of the deal, as was Thors return to Earth if there was further trouble of an Asgardian nature. Hence Fury’s assured comment at the end that “they’ll return when we need them” and thus Thors reappearance on Earth in “The Dark World” and his quick dismissal to the council saying "I didn't argue with the God that did." Not like Fury to give in that easily is it?
It would presumably be VERY easy to take Coulson back to Asgard or do any number of journeys once the Tesseract was working again in a short amount of time, hell they transported an entire Chitauri army to Earth with it so an Asgardian, a couple of people or a medical team would make little to no odds.
Healing Coulson, presumably if they had him on life support or in suspended animation could happen a number of ways. There are the healing rooms on Asgard (as mentioned in Thor), there is Mjolnir that restores Thor to life in the first film and could be possibly used on others and there is Idunn, Asgardian goddess of immortality ,whose not yet appeared in the films, but whose apples keep Thor in tip top condition and health.
I some what suspect we will get to see the actual answer of how Coulson actually came back AFTER “Thor; The Dark World” as, if the trailers are to be believed, there are a couple of scenes that suggest Thor takes some severe damage, and Jayne looks like she might possibly be in need of a reamination as well.