Thing is, I find it a lot easier to believe that going back in time, regardless the reason, does create a different, alternative timeline. Which is why I did have a bit of trouble accepting when Scotty and McCoy gave the transparent aluminium formula to that engineer in The Voyage Home. It would have made more sense to try and find the company which actually created the formula and maybe provide them with the knowledge a bit before it actually happened.
In the novelization, Scotty realizes he's met one of his "engineering heroes"--the man who invented transparent aluminum. Therefore, his "giving the formula" is rather like Spock Prime's giving the transwarp formula to Scotty in the new movie. Just a tangential bit FYI.