1.) There are no flat quotes -except for the port-airport journey- everything is metered and computarized in all taxis. There is no bartering either, fares are regulated, and and all taxis, regardless of the company they might be working for, will charge you exactly the same. All taxis in Barcelona are black and yellow, regardless of the company.
2.) Google Maps is not a good tool to predict traffic as it's often wrong, at least in reference to Barcelona. In general terms, the taxi driver will take you by the most efficient route -assuming it's one of the honest majority of taxi drivers in the city- as he knows when it's best to take this or that depending on the time of the day and on the potential road works and other "obstacles".
3.) For info, albeit technically is the B-10, no one refers to it with that name... it's the Ronda del Litoral. Yet this road is used by many trucks to get to one of the industrial ports of the city so it might be packed at certain times of the day, so sometimes is best to take the Gran Via de les Corts Catalanes -which later, way outside the city becomes the C-31. For info, in Europe, the roads tend to start outside the cities, so the "numbering" commences once it becomes a road. Before that, within the city, it'll be this street or that avenue and that's how locals will refer to it.
4.) My personal experience on the estimate in the journey from the area of Carrer Princesa to the airport is roughly around 35€ (day-fare)... again, depending on the traffic, of course. Also note that there are three fares: day fare, night fare and week-end fare and the cheapest (day 8m-8pm) is roughly 25% less than the most expensive (week-end, 8pm-8am). When you reach the airport, additional charges might be added to the total fare (ie. entry to airport and luggage bigger than a certain size). In any case, everything is computarized and it'll be shown in the printed receipt. Fares for 2016: http://bit.ly/BCNtaxi2016
5.) Most (not all!) taxi drivers do speak English -or at least mumble it- so you shouldn't have a problem communicating with them, even if the conversation is very 'simple'. In any case, just ask the receptionist at the hotel to convey whatever you want to say to the driver... that'll be the simplest way, won't it? :))
Enjoy!