You can be certain you can fire it arbitrarily, in the April Game Informer interview the devs were discussing the harpoon and said it has many uses, and that you can remove bits off other cars one by one. There's also a thunderpoon which is like the harpoons from Fury Road. This game is clearly very...
Harpoon is also limited to the Magnum Opus. To me it seems they're trying to emulate the action of the movies which relied very little on guns but mostly cars.
Max can only snipe from the back of the Magnum Opus. I would presume there are heavily scripted missions but from every article I've read (I'm trying to hit all of them) you have complete freedom for most of the game.
From what Avalanche has said Mad Max the Game is a completely separate entity to both Fury Road and to the original trilogy. It might be possible that WB Games has made them make adjustments to bring it closer to FR, but I'd think it's more to do with the game not being as far along as they had thou...