The dining plan is great! I believe that you must buy the plan equal to the number of nights booked in the room. As to availability with the discount, I don't know.
Check out Mousesavers.com which has a great section of "cheap eats". One of the tips is two slices of bread cost @$.75. In some food courts peanut butter is free, in others it is $.50. Jelly is free in all the food courts I have been in. So, for < $1.50 you can make PB&Js.
Also, think about splitting some meals. Two that have always worked well for DH an me. One is the 1/2 chix and ribs combo at the chicken station in Cosmic Rays in MK. It is only one counter credit on the dining plan and plenty of food for 2. It is a better value on the dining plan, but for about $7.50 each you will have a great, filling meal.
The other is the double burger at Pecos Bills. For an extra $.60 (two years ago) I purchased an extra hamburger bun and made 2 single burgers. This option should run about $4.00 each. This is even less that purchasing a child meal.
At Epcot - if you enjoy different foods, the Tangerine Cafe in Morrocco has Shawarma Platters for $12 - $14 complete with side dishes and bread that are enough for two to split. Don't overlook the refreshment port for free beverages and free cookies at the Nestle Junior Chef experience (opposite soarin' entrance in the Land) (just check the posted schedule and sometimes thay give out cookies to all spectators also, not just participants).
Lastly, check out the menus on allears.net and you will be able to better plan which dining experience will work best for you. Have a great trip