Is seeking forgiveness and making supplication sufficient after a religious discussion with non-Muslims about the tolerance of Islam, singing, hijab, and the destiny of humanity, given the speaker's fear that she may have conveyed incorrect information or misled someone?

What you mentioned about the disbelieving woman is generally correct, but the statement "Whoever harms a Dhimmi has harmed me" is a lie attributed to the Prophet, peace and blessings be upon him. The correct statement is: "Whoever harms a Dhimmi, I will be his adversary." As for your statement "we cannot be certain whether we will be in Paradise or in Hell," it contains an incorrect generalization; for whoever dies as a Muslim monotheist, we are certain of their entry into Paradise, and whoever dies upon a religion other than Islam, we are certain of their eternal dwelling in Hell. What you mentioned about the prophets being sent with monotheism, Islam permitting singing on Eid, the danger of polytheism, and that monotheist Muslims are hopeful for forgiveness, is true. We advise you to strive in learning and verifying before speaking on matters of religion.