a) I wouldn't bother spending only one night in London before taking off for Paris. Getting into town from- and back to- the airport will eat up time, etc.. I'd save it for the way back.
b ) You can take a bus from Heathrow to any of the other airports in the London area. Some airlines run a shuttle, but I don't think Easyjet does. Check out timetables & fares at http://www.nationalexpress.com/
c) You can skip the whole bus ordeal and take BMI directly from LHR to CDG. You can get a flight for as little as £11 - http://www.flybmi.com
One of the benefits is that BMI is a Star Alliance partner. So if you're flying into Heathrow on a Star Alliance airline (United, etc.) or an independent partner like Virgin-Atlantic, you can have your baggage checked through to Paris from wherever you originally depart. This is *very* handy.