No, you cannot control multiple teams in GM mode and there is no Single Season mode. And the computer makes awful roster moves and decisions.
This game is pure trash, I can't lie. My opinion of this game is that it is absolute garbage. It isn't even hockey. I try to be optimistic about it, but it just does so much wrong and so very little right. There are opinions and impressions all over these boards and about 70% of the people dislike the game, in my estimation.
I have probably played this, and every version, of NHL more than just about anyone on here and this year's version is possibly just about as bad as any of them ever have been. The game can't seem to replicate even the most simplest aspects of a real hockey game and strategy and hockey knowledge mean little to nothing.
I am coming up on my 300th online game and I am ranked in the top 75 and I hate it. I hate it when I win as much as I hate it when I lose. The only reason I even play it at all is because I am good at it and I like to win at things. That and because there is absolutely no other video game that is called hockey.
Offline is so boring, easy, and predictable that I would probably predict that the game wouldn't last even a month in your rotation of gaming if you like anything else in the world.
These are my opinions on the game. Every week we have people coming in here and asking about the state of this game. I can't stay silent and let a counterpoint not be heard anymore.
If you want more specific examples check out the bugs and glitches thread, the wish list thread, the somewhat comprehensive list of game play flaws thread, the impressions thread, or any other thread in these forums asking where things are or why some feature is broken or flawed.
Or just take the hint that these forums are nearly a ghost town other than a few diehards trying to brainstorm ways to fix the horrible and incomplete game that we have been served. Even the people that claim they love this game have sold it, become bored with it, or only play one very specific game mode over and over again and claim they can look past most of the flaws and just enjoy it for what it is because this is all we are getting.
Make your own decision though. But if you buy the game please come back in over a month and let us know if you are still playing it and what you think at that point. Because the luster of this steaming pile of **** wears off super quick, within days, for most hockey fans anyway.