I think if you try to get your reservation for Chef Mickey's on 12/26 as early as possible (time wise) and as soon as you're able, you might be able to get it.
The problem is that people are allowed to book as far as 10 days ahead (or to the end of their stay, whichever comes first). What this means is that people who arrive as early as 12/17 will be able to make reservations as late as 12/26, and Chef Mickey's is one of the most popular places. You'd also better hurry and make that Tusker House breakfast reservation as well, because that one goes pretty fast as well.
As an alternate on 12/26, my own recommendation would be Kona Cafe. It's also on the monorail line. Personally I don't care for 'Ohana (yes, that's how it's spelled)
For that open lunch, I'd sit down at Yak and Yeti.
Instead of trying to jam 3 meals in, try a late breakfast and then do dinner at one of the signature restaurants (Narcoossee's, California Grill, Citrico's, Flying Fish Cafe, Yachtsman's Steakhouse).
Garden Grill hasn't served lunch since October of last year.
Flying Fish or Yachtsman's would work if you're planning Epcot for NYE.
Also, even if Magic Kingdom is "closed", the midnight fireworks show is mirrored on Seven Seas Lagoon (it's a double of the in park show) and the 7:30 Illuminations is the Christmas version while the midnight Illuminations is the New Year's countdown. MK has an 8:30 showing of Holiday Wishes (inside park only).
Yes, you have 21 credits, and I suggest you try to not waste them on CS meals. You also have 14 snacks, which can be used to stave off the hungries to tide you over between breakfast and dinner should you decide to do 2 credit signature places.
I think Beaches and Cream is a waste of a deluxe TS credit, their menu isn't much different than one of the walk up hamburger stands and they don't have appetizers.
I would also suggest Biergarten and Tokyo Dining.
Another suggestion is Crystal Palace for either lunch or dinner. Sure they have the same caracters as breakfast, but they have a very large and varied buffet.
Oh yeah, good luck getting Le Cellier, that one is next to impossible, especially with the 10 day rule, and I personally feel that they've dumbed down the menu at Coral Reef so badly, that it isn't worth it any more, unless you're a first timer.
Check the menus for all of these places at:
Restaurant Dining Menus from Walt Disney World