Why can't I repeat the verb with degree complement in “昨晚我的朋友表达得很明确”?
Yes. 表达 isn't a VO, it can't be split like you're trying to split it.
No. 表 isn't used by itself as a verb like that. 


Also you could just opt for something much simpler like: 已经说得很清楚, rather than the mouthful that your current sentence would become.