Since you'll have a car, I think driving back to Frankfurt would be the way to go, since it's in Germany. The other option would be to drop the car in Cologne and train to either Frankfurt or Amsterdam. Cologne airport seems like it is the element that really drives up the flight times.
Since you're interested in extra legroom options, check out Delta flying back from either Amsterdam or Frankfurt instead of Cologne. Delta runs a non-stop 757 to Paris from Pittsburgh and then you connect to anywhere in Europe on AirFrance from there (except Cologne apparently). You could reserve economy for around $1450 and then upgrade the seats to "Comfort+".
You'd buy the economy tickets for Delta first and then select the Comfort+ seats in the manage my booking website. The Comfort+ seat upgrade for two people on Seattle-Amsterdam runs around $500 total roundtrip...maybe it's less on the PIttsburg-Paris route since it's 2 hrs shorter? The benefit of the Pittsburg-Paris flight is no customs at a busy USA airport. The downside is that it's on a 757, so it's not as spacious as a bigger wide-body jet.
Another option would be American Airlines: American to Chicago and Air Berlin on to Berlin. Home from Frankfurt to Philadelphia and on to Pittsburg both on American. I'm not sure about the extra legroom options on those transatlantic options since I haven't flown AA or Air Berlin to Europe myself.