The meaning of Let us not or Do not let us

As far as I know, the negatives of Let us are Let us not and Do not let us. And Let us has two different meaning: one is a suggestion and the other is an imperative. Then doesn’t Let us not or Do not let us have the two different meaning?

a. Let us not watch the movie.
b. Do not let us watch the movie.

I think both a and b are correct and have the same meaning: one is How about we don’t watch the movie?(suggestion) and the other is You shouldn’t allow us to watch the movie(imperative). Am I right?