你的分析都是对的
关于时间状语yseterday.因为状语是对句子的补充说明,那么不要的话只是意思不全,对吧.
那省略when his father came yesterday--->He was doing hie homework,无歧义.
省略when his father came ---->He was doing hie homework yesterday.(他昨天一整天都在做作业?)
看出问题来了吧,所以yesterday这里只能是状语修饰came 而不能修饰was doing.