Is it within the rules of the game to reverse an offsides decision? A linesman calls for offsides and the team that had the offsides call against them swarms the linesman. After a couple of minutes, the linesman reverses his decision, which results in a goal.
What is the sequence of events? If the referee blew the whistle for the call and the AR immediately realizes his mistake I believe the only thing you could do is have a dropped ball instead of the IFK (I'm not 100% sure about this though). I'm confused by your scenario where he changes his mind minutes later, and somehow a goal is scored. Once a call is made and the game is restarted your decision is final, you cannot change a call later in the game even if you realize you made a match critical mistake.
The call is the referee's to make, not the assistant referee (linesman). The assistant's flag is for the referee's information, it's the referee that blows the whistle to stop play for an offside infraction based on that information. If the referee did not stop play by blowing the whistle before a goal is scored, then makes an offside call, he may reverse that decision based on any new information up until the point that play is restarted (kickoff) or the period of play (half or game) has ended, and award the goal. If he blows the whistle for offside before the ball is in the net or otherwise out of play, he may reverse that decision based on new information again up until play is restarted or period is ended, and in this case the restart would be a dropped ball for an inadvertent whistle from the location of the ball at the time play was stopped. In either case the reversal should be based on a discussion and sharing of information between the referee and the assistant(s), not from protests by players or anyone else.
And as long as we're quibbling, note that a timely change of decision does not not necessarily have to be based on "new information;" it can also be based on simply waking up and recognizing that we got it wrong. And I suppose we might also note that any "new information" should only come from other members of the referee crew (and not, e.g., from the coaches or other players or the sidelines or the Jumbotron).
So a player admission and the referee reversing a corner kick in a Serie B results in a post-match green card fom the referee. I guess a novel approach to sportsmanship: http://the18.com/soccer-news/player-shown-first-ever-green-card-italian-serie-b-match