-What is the meaning of "mushrikat" as used in 2:221?
According to the Quranic terminology it means the terrorist one , and /or the one who support terrorism. Terrorisim ( or killing the innocent people ) in Islam means ( Shirk and Kofr ) or infidelity.

Does the Quran really forbid a Muslim woman from marrying People of the Book?
Any peaceful human is Muslim regardless of his faith or belief.
If the Christian (or the Sunni Muslim) is terrorist he becomes infidel according to his behavior. So, it is prohibited to marry him or her.

- If I marry someone who is non-Muslim, am I no longer considered to be a Muslim?
You are Muslim as long you are peaceful in your behavior, and you are Muslim in your heart as long you believe in God alone, without any idol with Him.
Being Muslim in behavior and /or in heart has nothing to do with your marriage. Every human is responsible for his own deeds and belief. As a woman, you are independent human with and without husband.