Using "let" in imperative sentence

Well it depends on what you are trying to say, if you are using imperative you dont have to use the "will you" at the end, because is an imperative order!

is not like saying: "it´s a beautiful day, isn´t it?"

And about "Don't let us stay here, shall we? "

You are ordering someone to not let us (we) stay here, shall we is used to talk about when we are part of the group of people you are talking about.

Example: "Let´s go to the mall, shall we?"

"Don't let us" in that sense is a BrE way of saying "Let's not." It's not really imperative, but is more of an entreaty.

Usually you hear it in contracted form:

Don't let's tell Andrew about this anytime soon.