At the end of the day (no pun intended) if Swift cannot sing the song as written, the real built in safety net is to have someone overdub her voice. That's the true built in safety net for ALL movie musicals and a similar technique is used in live productions. Audrey Hepburn, sang and recorded tracks for My Fair Lady
which were not used. Rita Moreno did not sing A Boy Like That/I Have A Love
in West Side Story
. And on stage, the actress playing Christine in The Phantom of the Opera
, prerecords the highest, and final, note (E6) in the song The Phantom of the Opera
My only real point in choosing those two versions of I Want to Hold Your Hand
, was to show that it's not just the lyrics, the speed of the song does do a lot to set the mood.
ETA: My favorite is the 10 Year Anniversary Concert. I want The Complete Symphonic Recording.