In answer to your first question, Circe is mainly a danger to Odysseus and his crew because she and her underlings are a distraction. Odysseus cannot waste any more time getting home. The reader discovers what has been happening to Penelope and Telemachus while Odysseus has been absent, making Odysseus's soon return even more significant. Circe is primarily a danger to Odysseus not just as a distraction but also as someone who takes away his focus on and devotion to his wife.
Most would argue that Circe is more of a danger than Polyphemus the Cyclops. First, the Cyclops is easily recognized as a danger by Odysseus and his men. Circe is a subtle danger that many characters do not even see until it is too late. Moreover, Odysseus and his men are trained for handling physical dangers such as Polyphemus, and Odysseus easily outsmarts the giant. Circe, however, represents a psychological and emotional enemy who is armed with wit and beauty. Odysseus especially is rusty when confronting such a danger.