1,by which time it wil be completely finishied是定语从句. by +时间：到什么时候的意思. which time 代替next monday.
2, which I think ____ (be) reasonable这是个定语从句修饰idea,I think 是插入语 所以可以看成which ____ (be) reasonable ,which代表的是the idea 作定语从句的主语,所以系动词be只能用单数is或was, 而I think 表明是一般现在时所以用is