Although she saw so many people_____in front of her,she looked calm.

seat n.和vt.
做名词时用法：take a/one's seat 坐下来，就座
如：Take your seat ，please. 请坐。（平级之间）

作动词时及物动词，后面必须有宾语或用其被动语态
如：Be seated,please. 请坐。（很礼貌的用法）
seat sb. 某人就座 seat yourself ,seat him

sit vi.不及物动词，不可以直接接宾语
如：Sit down，please. 请坐。（有命令语气）

所以不可以换

seated 表示一种状态