Is what transpired between me and the young man, in terms of offer and acceptance via chat, considered a valid marriage contract? And are those who were in the chat considered witnesses? Am I married or single? And must I divorce this unknown young man before marrying a new suitor?

A valid Islamic marriage (Zواج شرعي) has conditions and pillars without which it is not valid, including the guardian (wali) and witnesses. What occurred over the internet does not entail the rulings of marriage, and he is a stranger (ajnabi) to you, and he is not obligated to divorce you. Repent to Allah and do not pay attention to this matter. If someone whose religiosity and character you approve of proposes to you, do not hesitate to accept him.