What strikes me here is that if the OP had have told queue guard that they was travelling in First would the guard have asked for proof of that as well?

I have never been asked for proof of my eligibility to join a queue when travelling in First. They tend to believe you when you say you are in First so why shouldn't they when you say you are Emerald? What is the difference? Either way you are going to get caught out at the desk and should be sent packing if not entitled to be there.