You're not going in FC by any chance !
1.
AFAIK all documents are the original - if you are who I think you are, just take the brown folder from the Squadron - should have everything you need
(C of A, insurance, noise certificate etc). Don't forget a copy of the interception procedures.
Safety Sense Leaflet 11: Interception Procedures | Publications | CAA
is sufficient for this.
2. Yes and yes - actually I don't think NW close the flight plan, they simply don't initiate any overdue action.
3. I stick with Manston till mid channel, then tend to go straight to Le Touquet. Lille don't really care about you. I've also gone straight to Manston after leaving Le Touquet zone on the return leg. You could go to Lille, but again they won't be particularily interested in you. Lille is class E so there's no requirement to do so.
For the GAR, you can give it to the nice people in the office who'll do it for you. And I think the requirement to email Le Touquet has been shelved...