So I fought back a little on the key change debate because that's editing the instrumental, not the vocal. But putting together different pieces of a vocal is essentially editing your performance and not giving a true representation (the term for this is "comping," by the way, which is a step people take when mixing a song).
You have the opportunity to do as many takes as you want in this competition, just pick the best one of those. If you can't give a good performance in a single take with a whole month to prepare and re-record, honestly you probably don't deserve to move on anyway. *Prepares for backlash*