Easyjet makes money on excess baggage, so yes you'll if the checkin agent is paying attention you'll be paying for 10kg of excess.

From what I understand the only way to avoid these charges when you connect in Europe is to interline your luggage from the US to your end destination. That basically means the connection must be within 24 hours and the connecting airline needs to have an interline agreement with your airline from the US. I doubt if Easyjet has an interline agreement with anyone, but I could be wrong on that part.

On flights within Europe (and much of the non-American world) 20kg is the norm for Y class. Some airlines are a little more generous. 30kg is the typical limit for J class. I don't know how people manage to go on a long vacation and pack anything but we are in the minority on the 2 bags at 40lbs each (or more).