Callahan and McDonagh are, as previously said, very likely. But the best way to tell is to look at the competition.
Lets look at the 2012 squad and who might compete at each position...
Center: Stastny, Kane, Pavelski, Kelser will all be in their prime. Stepan would have to beat one of them, unless Pat Kane was deployed on the wing.
Wings: USA strength. Ryan, Brown, Kessel, Parise, Backes are the locks. 5 guys. Cally makes 6. Malone, Langenbrunner can be upgraded on. If Jaime is even playing then. Drury is obviously retired. There is a spot for Kreider here if he becomes a 30 goal guy. Maybe. The U.S. has a lot of good young wingers.
Defense: Suter, EJ, JJ are locks. Rafalski is retired, Gleason and Orpik might fight for a 7th spot, but I think that McD will, barring taking several steps back will absolutely be on the roster. Probably top 4.
And this isn't even including new guys that have broken the scene more recently like Cam Fowler.
Unless someone absolutely explodes into a premiere defenseman in the near future, McDonagh is team USA's #2 defenseman. He's better then EJ, and about twice as good as JJ.
Callahan is a lock. McDonagh has a very good chance amongst a deep group of d-men. I think Stepan has a very good chance as well, as the U.S. doesn't have a lot of depth at center (at least skilled centers).
Erixon can't play for the US under IIHF rules as he's already played for Sweden.
A common misconception; not true.
Change of national eligibility (The ‘four-year’ case)
A player, who has previously participated in IIHF competition, can switch national eligibility (but only once in a player's life) if:
1)He is a citizen of the new country of his choice
2)He has participated for at least four consecutive years in the national competitions of his new country, during which period he has neither transferred to another country nor played ice hockey within any other country and has not played for his previous country in an IIHF competition during this four year period.
3)He has an international transfer card (ITC) that shows the transfer to the national competition of his new country and which was approved and dated at least four years before the start of the IIHF competition in which he wishes to participate.
The Nedved scenario I assume was different being that he never decided to go any of those routes and just stuck with Canadian Citizenship. Never saw him play for the Czechs, and I don't remember seeing Holik on any of their teams either. At least in Olympic play.
I doubt that Erixon would prefer to play for USA instead of Sweden. He is swedish and raised here, he was born in N.Y and will be playing in NHL by then, sure. But his parents are swedish etc, he grew up here and I think he consider himself as a swede. Easier for him to get a chance in sweden then it would be on USA's team.