Is there anything upon her for her initial assumption that "al-tashqir" (lightening/bleaching hair or eyebrows) was forbidden, and her subsequent avoidance of it, then her change of mind after learning of its permissibility?

The fatwa is that it is permissible to lighten eyebrows (tasheeqir) as long as it does not involve deceiving a suitor or imitating disbelievers. Your intention to stop lightening them and then your change of mind does not entail any consequences. However, it is more scrupulous to refrain from it to avoid disagreement.