Just a couple of notes. I don't think I saw answers to a few of the questions you asked. Also, some unsolicited advise.
First, no one's going to ping you for posting your actual cost estimates. That's not a violation of board rules or ettiquete. In fact, it's helpful to see other's math on these things when making your own decisions. (It also helps others to see if there's a flaw in your calculations and make recommendations.)
You mentioned that some of your party want to be on the plan and others don't. This will only work if everyone who wants to be on the plan is on one reservation (in the same room) and everyone who doesn't want to be on the plan is on another reservation (in other room(s)). Disney's DDP rules state that everyone on a package reservation must have the exact same package. If selecting the DDP, it must be for every person on the reservation for every night of the reservation. Also keep in mind that the DDP is accessed using your Key to the Kingdom. This has your room key, DDP, park ticket, and room charging (if selected at check-in) all on the same card. Each person gets one and they have the name printed on them.
Just want to make sure you understand the flexibility of the DDP. While you receive one TS, one CS, and one snack for every night of your reservation, you can use those credits any way you choose from the time you check in until midnight on the day you check out. You don't have to use them one each day.
You also don't have to use your credits so that each person uses their exact number of credits. If someone in your party gets hungry faster than someone else, let them use an extra snack credit or an extra CS credit.
Believe me, we're not kidding when we say the DDP is a lot of food. I'm a big, big eater. Even so, we had trouble using all our credits by the end of the trip. Many others will tell you they've had similar experiences. TS are easy to plan because you have to make the ADRs. CS and snacks can build up. (We'd usually split a CS meal between us because we were still so full from a TS meal. That left us with an extra CS to split as a late night snack or as a breakfast. Even so, after two weeks on the DDP, we had 8 CS credits and 10 snack credits to try to use between us in the last three days.
You asked about room service. Room service is now available with the DDP, but ony if you're staying in a deluxe resort. Room service costs two TS credits. (Ouch!)
A snack can go a long way. The resort gift shops have already been mentioned. At Saratoga Springs Resort, there was a little grocery area. A bag of 6 bagels, a box of donuts, a family-size bag of chips, were all great uses of a snack credit. They lasted all week. In the parks, french fries or onion rings are often available for a snack credit.
We like the DDP more because it allows (forces) us to try new TS restaurants. If we weren't on the plan, we would probably eat many more CS meals. We also like having everything paid for up front, and not having to worry about running up a bill that will be too much to handle after we get home. Of course, we have a small budget for OOP things, but the majority of the food bill is already covered.
Another benefit of the DDP if you have older kids who will be doing some of the theme parks on their own, is that you can let them use their DDP credits without having to put room charging on their room key or having them carry cash to buy their own meals or snacks.
Keep track of what credits are being used, how many are left, and how you plan to use them. Each time you purchase something on the DDP, your receipt will show how many credits are left -- but only for that type of credit. For example, a TS receipt will only show how many TS credits you have left. Your resort concierge, however, can print you a full listing of where you've used every credit that's been used and how many credits total you have left.
If you're looking at the DDP for purely economical reasons, there's an important thing to keep in mind. The DDP will difinitely save you money if you plan to eat in at least one TS restaurant each day, and if you plan to order the amount of food offered on the plan. For example, the DDP gives you an appetizer, entree, and dessert for each person. If you were going to a restaurant without the plan, would you order an appetizer, entree, and dessert for each person? Would you maybe skips appetizers or share one for the table?
My #1 DDP tip: When ordering a drink at a CS as part of a meal or when using a snack credit to get a drink, always specify the largest size available. If you don't specify a size, they'll usually automatically give you a regular instead of a large. Say, "I'll have a large coke," instead of, "I'll have a coke."
An alternate to the DDP to look at is the Disney Dining Experience (DDE). This is available to Florida residents and annual pass holders only. You have to buy the DDE at $60 for one year. However, it gives you 20% off the entire check (booze and all) at select restaurants. If you're just looking at the DDP for budget purposes instead of the other benefits it offers, DDE is something to check out. It may be the better deal. Only one person in your group needs to have a DDE card (so long as they're going to be present at the meals, you'll be paying on a sigle check, and don't have more than 10 people). Do the math. It would require upgrading one of your park tickets to an annual pass, then buying the DDE card. (You can upgrade the ticket you get as part of your Magic Your Way package to an annual pass by simply paying the difference between the two ticket costs once you get to a theme park.) You can only purchase the DDE by phone is the only wrinkle. I don't know how long after calling it takes before you can pick up your temporary card. See
this link for more details on the DDE and a list of restaurants where it's accepted. (We've done DDP twice, and will be trying DDE on our next trip for comparison purposes. Many DISers swear by it.)
What good news. I thought all character meals were "signature" dining experiences.
So....
Looks like dessert for breakfast for a couple of days! 
and still don't have it down pat. We are a family of 4 and have the following ADRs. We are only doing the DDP during our 5 night stay at SSR. We also have a day at the beginning and a day at the end but chose not to add the DDP to these two ressies. Figured we would have more than enough CS to cover our last ressie and would be cheaper to pay OOP for the first one.
