Is it forbidden to inquire about marriage in this manner, and is it considered incitement to discord, and is marrying her after her divorce from her current husband a sin?

If you did not propose marriage to her, and you were unaware that she was married, then there is no sin upon you, and you are not considered a 'mukhabbib' (one who corrupts a wife against her husband) due to the lack of intent. Errors are legally excused. If she is divorced from her husband, or if the Sharia (definition) judge annuls her marriage, then your marriage to her is a good matter if she is a woman of religious commitment and good character, provided you are able to act justly between her and your first wife.