Hi,
In comparing Gare du Nord with places you are familiar with, I would rather walk through that area solo once the sun goes down than doing likewise at Seattle Pioneer Square. You know spots in the Bay Area...choose between Gare du Nord or 13th/Broadway in Oakland at dusk, or other similar spots.
I always stay at Gare du Nord, nice hotels from 2 to 4 stars, just depending on your level of luxury, mostly since it's so very convenient for arriving in Paris, either direct from SFO to CDG after a ten hour plus flight, or from London via the EuroStar, as you're doing and departing from Paris. Nord as a station is much better now than it was 30 yrs ago.
True, relative to other nationalities American tourists are far fewer in numbers at Gare du Nord or you just see them passing through. Those descriptions you list above I would consider part exaggeration and part valid. The gypsies often operate at Nord as do the scammers with their petitions, "do you speak English?" signs. Ignore these types, be aware if you see them from afar and ditch them. I would be on guard at Nord as in any train station in Paris...Gare de Lyon, Gare de l'Est (I like that one much better over Nord)
I see Gare du Nord as a practical ( a bus, ie no need to rely on only the Metro), RER, Metro, EuroStar, train stop centre), convenient, basically a "get in, get out" place, unless you want to explore the place, really get to know it inside out, which I've done there too. Are you staying at Nord or just passing through arriving by EuroStar? Before meeting up with tour group, where are you staying in Paris? There are hotels (good to expensive) right across the street once you exit the station.
It can be daunting at Nord for a first timer since it's a crowded station with people coming off the EuroStar, the TGV and regional trains from northern France (Amiens, Arras, Soissons, Dunkerque, Abbeville, Lille, Bethune, Cambrai, etc) and the RER from the adjacent station.