lol, coming from the English major.

Agreed, anyway, it's a valid phrase but not something a native English speaker would typically use. 'at once', for instance, is not something most people would EVER say, regardless the sentence or meaning.

It depends on who you are. Or rather, in what context. For instance there's a dude at work who fucking around alot recently, and if my boss tells him to do something which he doesn't do, it's not uncommon for her (though she just demoted her self, no idea who it is now) to say "When I say do *something* I mean do it at once!" But then, It's more common for her to say "Right away!"

It doesnt imply impatience at all. Its quite a formal and precise phrase that also is very forward and no-non-sensical.

Since i must be one of the members that knows least about english:

Shall is polite i thought, rather formal.
At once does imply impatience imo, it sounds rather strong.

So I wouldn't use these 2 in one sentence because it contradicts, you're not going to ask nicely to a superior for example to move his lazy ass. It could sound impatient or polite, depending how you put the stress in the sentence. But i think it just puts 2 too different emotions together.

I know. But its true, and its a language question, not a literature question! (yes, i know language is written too but shush!) anyway, point made now