Is "Lord Protector" just a professional title, and Corvo isn't a nobility, just some common (perhaps, even low-born?) bodyguard? Or it is a title of the highest importance, like that of real historical figures such as Edward Seymour, Duke of Somerset, Richard Plantagenet, Duke of York, and other Lord Protectors in the history of England?
Since Gristol is heavily inspired by England I believe it's the latter.
I've always assumed that Corvo belongs to a very high nobility because of his "Lord Protector" title. After all, in history of England this title was used for nobles exercising regency while the monarch was unable to rule (e.g., too young, ill, very old). I assume, in normal circumstances if the Empress was absent or dead, Corvo would become the Regent until Emily is old enough to rule (and he takes this position in the good ending of the game). In this case he is not just Lord Protector of the Empress or the royal family, but he is the Lord Protector of the State.
That would also explain why it is Corvo who was sent to the diplomatic mission of critical importance. There is no sense in sending personal bodyguard or even head of the royal guards, no matter how much he is trusted by the Empress, - in fact, it could be even treated as an offense by foreign governments: you don't send someone as a diplomatic envoy unless they're of sufficient rank. But if the second important man of the state presents his country while trying to negotiate some favors - that would add a lot of weight to his words. That's why the Empress HAD to send Corvo, not someone else, though very reluctantly, judging by her letter.