Flying Blue is a poor redemption value for Amex miles. I would perhaps recommend you look into doing JFK-MAN, or JFK-LGW with VS (Virgin), as the redemption rates are much better. Then get a connecting flight to CDG from the UK (costs peanuts anyway - for that you can probably use FB miles which are often better value short haul, otherwise EZ-jet is your friend)
Also if you have a FB card with 0 miles and that you never used, and transfer Amex miles to it, you will probably get blocked for a few weeks without being able to book your flights (a well known problem on this subforum - Irishguy already mentioned it above).
Your strategy of going J on the redeye and W on the return is a good one. Premium Eco is a decent class for a 7 hour day flight. Much less for getting sleep. Or for longer flights. But hopefully if you find a good redemption for your miles you may be able to go full J.
Safe flights!