吃午饭了, should 了 be right after 吃？
First of all, 
“吃午饭了” is usually used for "someone" to ask "someone" to have a lunch. Besides, it also can be meant that you already ate the lunch. 

While "吃了午饭 ” just mean that you already ate the lunch. 

Plus, there is no such thing like "吃了午饭没有“ It should be "你吃午饭了吗？” The "吗” is used for asking something.