Is it permissible to read the Quran for the deceased, or is it obligatory to supplicate for them only? And what is the ruling on a group participating in a Quranic recitation circle to complete the entire Quran by dividing it amongst themselves?

There is no harm in reciting the Quran and dedicating its reward to the deceased, but supplicating for the deceased is not obligatory. Gathering to recite for the deceased is an innovated matter that has no basis in Islamic law. It is better for a person to recite individually and dedicate the reward to the deceased if they wish.