Am I entitled to demand to spend more than three hours with my husband on Eid al-Fitr, and what is incumbent upon him to do on Eid al-Adha, especially since I agreed to marry him while he was already married?

The husband must be just in dividing his time between his wives. There is no harm if it coincides with one of their Eid days. A man can spend the first day of Eid with one wife and the second day with another. He is permitted to visit the wife who did not have the Eid day allocated to her during the daytime, without prolonging his stay. The Sunnah is for the husband to stay with a virgin for seven [nights] and with a previously married woman for three [nights], and then to equalize the division. The minimum turn for division is one night for one night. It is permissible for it to be two or three nights, even without their consent. However, it is not permissible to exceed three nights except with their consent.