If the job is worth keeping, of course you should fight it. I'd probably fight either way just to fuck with them and waste their time.

Yes, if it comes back negative dilute they will more than likely re-test you no questions asked, unless the collector noted something on your file/paperwork.

Were you getting the vibe that she thought you were trying to tamper with the test?

Relax bro, there are many many worse things that could happen to you, a year from now, you won't even remember today, unless you try really hard I wish I could tell yo uto burn one, but given the situation it doesn't seem like good advice. Go buy a bottle of rum and before you know it it'll be tomorrow!