4) I've heard that French authorities require you to insert a waypoint on the flight-plan to specify the point at which you will cross the FIR boundary. What entries are valid or can you only specify a known point such as DEVAL or RINTI?
You use the FIR identifier, and there is a specific format. If you intend to enter the Paris FIR 1 hour and 10 minutes after departure, write EET/LFFF0110. If you intend to enter the London FIR 15 minutes after departure on the return write EET/EGTT0015.
Didn't you write somewhere that you operate out of Cambridge or am I getting mixed up?