I always found that filing my flightplans exactly 120hours prior to COBT worked especially when you coincidentally filed your flight plans to give you an ETA that landed within the first 6 mins of each hour as they are limited to 10 IFR mvmnts each hour
so FP time 1230 and an EET of 0134 seemed to work even on a Friday afternoon. If slots were issued they were for the enroute segment and not the airport itself.
Spent every summer weekend there in 2003, managed to have it all worked out by August just as the season ended....
Of course if the boss gives you an hours notice to go on a Friday afternoon then you are fairly screwed. We would go to Nice instead We could then position from Nice to Cannes VFR via SW in around 10 mins on a saturday morning.
We would also use Helisecurite the Heli company who would normally come up with a free txfr if you bought 10 up front.
LFTH is not without its problems, its mil and the PPR is never available in a hurry.
WRT Reverse if yo need it use it, they would never prosecute somebody for using it if it was required for safety. Says so in the Jepps so must be true
All this is a couple of years old