①强调句 从 把it与that去掉后" You found the old man seated and dressing himself on the bed yestrday."仍通顺 可以看出.
②could it have ...是对过去的推测,不过这里是一般疑问句.
③seated没有用错,be seated=sit ,seated其实本身是个形容词,是后置定语,也可看作过去分词作后置定语,而此处“seated and dressing himself on the bed”是found的宾语补足语.
④翻译：有没有可能是昨天你发现那个老人坐在床上自己穿衣服?