quick rules question: if one were to cast this to invalidate the part of your backstory to which this spell is relevant, does that lift the restriction on others using it?
Technically anything your character does becomes a part of their backstory in the long run, so having this spell does make it part of your backstory. If another caster has this spell and targets you with it, they can target and redact specific rules (or words!) from this spell.
After they do this, you also gain an unaltered second copy of this spell as per its text.