I was going to suggest "Nothing but a viking helmet strapped to your crotch" but that violates a couple of your constraints.

Hmmmm.

At least they won't see the shrinkage.

Dress shirt, pants, coat, Viking Helmet.

Borrow a briefcase overflowing with bloody documenting.

You are Yorn McOalf, Viking At Law.

In case it's useful, I give you my own minimalist Halloween costume for this year:
- black trousers (or possibly skirt), black vest, black shirt. (I might substitute a flowing blood-red shirt.)
- a subtle black, scaly tail sewn to the waistband of the trousers. I made the tail from an old piece of electrical cord, cut to desired length and wrapped in black duct tape. The barbed end of the tail is also black duct tape.
- teeny tiny horns crafted from Sculpey and painted with metallic copper paint. I'm stringing them on beading elastic (but you could use any stretchy cord) and wearing them tucked inside curls of hair. Someone with shorter hair could do with even tinier horns, for subtlety's sake.
- Long nails, just brushed at the tips with dark-red nail polish, to give the look of scorched or crusted blood.

I'll also be wearing heavier than usual eye makeup and dark lipstick, and lowlighting the contours of my face and neck with dots of either eye shadow or lipliner, depending on which gives a better effect. (If you wanted a green scaley effect, you could buy a cheap tub of green clay facial mask and apply it sparingly. I did this the year I was Medusa, and the effect was startling.)

I had thought about carrying a briefcase full of contracts in a complex, scrolling font, but decided that Halloween is my night off from soul-wrangling.

Possibly: a nametag* reading "Pleased to meet you! Hope you guess my name."

*Yes, I did suggest this in an AskMe.

Yea, I'm voteing for Yorn McOalf, Viking At Law.

The Whelk has it.

For anyone else struggling, if you have an array of clothing available, maybe some hats or wigs or access to make-up (or an SO's or housemate's make-up), you can just start dressing up an hour before you leave, and decide who you are when you look in the mirror right before you step out the door. I've done that a few times and had fun.

Consider giving your Viking Lawyer a third name: Yorn McOalf _____, Esq. You might consider Lögsögumaður.

[/Scandinavian studies geekery]

Wear dress shirt, pants, dress coat, and Viking helmet. Go buy some chocolate coins. Add ribbons. Make sash. Hand out medals for inconsequential things like good diction. Ta-da! You're the Nobel Prize Committee.

Not my idea, by the way, but I can't tell you where I got it from.

If you don't go with the awesome viking outfit just wear everything else, shave, and buy a cheap fake gun somewhere. Instant gangster.

I'm rolling with Elsa's devil idea. I'm the Devil's Advocate. I have a nice briefcase with some quickie contracts for souls in them. I did the lemon juice invisible ink trick on them so if somebody signs them, I just hold them over a flame and a pentagram appears.

Thank you!