Yes, then Teixeira returned to the Yankees to see if they would up their offer, which they did. This has been covered many, many, many times - the Red Sox were not getting Teixeira. The plan, if they ultimately did somehow get Tex, was to trade Lowell, who still some had some value left at that point. He would have flipped to SF or maybe the Dodgers.
Because Lowell could've and would've been dealt in that scenario. There are several NL teams that could have used a bat at little cost (in prospects, Sox probably would have eaten a bit of his contract as well).
Guzman, would be untradeable. At this point in his career, he isn't great at either side of the game (batting/fielding). He's a complete waste of money. Lowell still has a good bat. Gonzalez can still play defense. Guzman isn't great at either.