After my tour in UK, our plan is to take the eurostar to Paris
arriving at Gare du Nord station around 4pm. Hope to save some money
by taking the metro to the airport, store luggage there, go back to
the city to do some sightseeing, return to the airport around 10pm,
spend the night at the airport until the next morning, go for
breakfast, then early check-in for the 12 hour international flight
home at 11am. Has never spent a whole night at an airport. Is it going
to be tough?

Prior to UK tour, we had already stayed in Paris for 3N so our
thinking is: why check into a Paris hotel for one night just for a
soft bed to sleep on but having to pay another US$80.

Is there any storage facility at the CDG airport and what is the rate
like? Are there comfortable seats at the airport for people to take
catnaps?

Let me add to my previous post.
Bring Eyeshades and ear plugs, airports are always very bright and the floor
polishers allways polish at 3am. Security wise, use your carry-on as a
pillow and try to improvise a way so that all other luggage is tied to a
limb so you will be alerted by any 'tea leaves'.
At CDG, you will be lucky to find benches without armrests, so go for a
carpeted area away from any doors, I used to find under escalators a good
place to kip.
You may also be better off with a sleeping bag as CDG can get mighty cold at
any time of the year.
Things may be be different now, I used to sleep all the time (about 10 years
ago) (apart from Dublin last year when I tried to find a hotel during the
'All Ireland Gaelic football') at airports to save a few quid while
travelling, and never had any trouble. As I said previously, check out '
http://www.sleepinginairports.com '.