Shawn could have completely changed this by saying, "gee, you might be on to something, I just reproduced the problem by visually looking at identical model phones and your -db is substantially lower, a hybrid PRL and a master reset isn't going to help you, let me see what I can do to get a replacement for you."
OP, I can definitely relate to your frustration, and I myself had to deal with a motley crew of imbeciles in getting my phone to me.
What I can tell you about people like Shawn is this: they really don't know anything about the phones they are representing. They are likely given a brief inservice which includes a set of protocols, and he was simply going through his protocols. His superiors did not provide him with extensive trouble-shooting advice beyond those protocols. This is the unfortunate reality of dealing with big electronics stores. Gone are the days of individual specialty stores where the personnel really really KNOWS the products they are selling and have the ability to provide dynamic advice on the scenario you present. Today, Shawn is providing generic help on the newest, high-demand cell phone. Tomorrow, he'll be highlighting Mario Galaxy 2 for the wii.
I know that doesn't lessen your frustration, but hopefully it can help you realize that Shawn is not the person you go to for real technical help.