How authentic is the attribution of the verses, "Would that You be sweet, though life be bitter... And all that is above the dust is dust," to Rabi'a al-Adawiyya, and are they considered a violation of Islamic law and a display of ill manners towards God?

These verses do not belong to Rabia al-Adawiyya, but rather to Abu Firas al-Hamdani. Their content is sound if addressed to Allah, as Ibn al-Qayyim indicates that the rectitude of the soul is achieved by preferring Allah's pleasure. Abu Firas excelled in the general meaning, but he erred by uttering them to a creature.