When Ted and John sit in the park and attempt to smoke pot, the bench is right next to the tree trunk. After they are approached by Donny and his son, they are shown again sitting on the same bench that is now several feet to the left of the tree trunk.
No. The screenplay is originally based on an idea by Seth MacFarlane.
What are the differences between the R-Rated and Unrated Version?
The fans already know the procedure from MacFarlane's shows, so it's not a surprise at all that his first motion picture is being handled in a similar way: besides the Theatrical Version, the Blu-ray and the DVD releases in the UK and the US also contain a so-called Unrated Version (name in the US) or Extended Version (name in the UK). The Unrated or Extended Version is 6 minutes longer. As expected, the new footage doesn't reinvent the wheel, that's for sure. And there are no scenes that were censored in the Theatrical Version either (like the F-word in the TV Version of Family Guy). Refering to the rampage at Virginia Tech University might be borderline but then again gags like that are on MacFarlane's shows all the time. Probably a highlight is the scene in the beginning when Donny (as a kid) takes notice of Ted for the very first time. Apart from that, the longer version contains some nice gags that enhance the quality of it. Due to the use of alternate footage for some scenes, the Theatrical Version is still worth being watched because some gags from the Theatrical Version have been removed in the process of editing the longer version. Finally, fans don't get around watching both versions anyway. A detailed comparison between both versions with pictures can be found here.
What is the first song?
"Everybody Needs a Best Friend" by Norah Jones. The complete list of songs can be found here with their corresponding scene descriptions.