完全正确!
Who is /was the compere?这里the compere(主持人)是主语,who是表语.
I don't know who you are.这里you是主语,who是表语.正如你理解的那样,如果who是主语,按照宾语从句中疑问句用陈述句语序那就该是：I don't know who are you.
不过以下句子中what做主语,这个是英语习惯用法,也是重要考点：
I don't know what is the matter.
I don't know what is wrong (with him).
即what is the matter中what就是主语,what is wrong这个what自然也是主语.
欢迎探讨词法语法问题.