Yup, I sure did say fire away and I meant it. The only problem is that most of the time, I am NOT at all good about making a long story short.
The only table dining restaurant we ever ate in the MK was inside Cinderella's Castle. You can still eat there now, but it's become a little bit more of a show dinner with a fixed menu (like choose 1 of a few apps, entrees and desserts. From what I understand, the show involving princesses is quite brief, they don't come around to the tables for pictures and although some people have loved it, I think I've heard more complaints than anything so far. It may take up more time than you're willing to give up. In addition to the that and the Crystal Palace, which you aren't interested in, there's the Liberty Tree Tavern, which has an all you care to eat meal, served family style. This used to be a character dining experience but it isn't any longer. I know you said that isn't a priority, though. There is also Tony's Town Square serving Italian, right up in the front of the park and The Plaza Restaurant basically at the end of main street serving things like burgers and sandwiches. (see menus in the link below)
Now, you visited once before 3 years ago. It is possible you may feel quite whirlwind all over again, especially with 1 day in the MK. I would be prepared for some hefty crowds. I have been to WDW hundreds upon hundreds of times, at all times of the year but crowds can be the strangest thing. It's so hard to say, but I am guessing it won't be any kind of slow day. Last year we stayed at the Caribbean Bch (even though we're locals - and I LOVE that hotel btw) and tried to do the parks in mid-October when it should have been a good time and it was so crowded I couldn't believe it! Halloween Day/Night at the MK tends be be crowded, perhaps because of its' festive feel for the day. My first two trips to WDW back when I was living in NY, I went October 1989 and then May 1992. Not even 2 years in between and I still had that "so much to see, so little time" feeling. Not everyone feels this way, but be prepared that you might. There's that, plus the fact that with children, or even adults, aging a few years can give a different perspective on the parks and alter things that way.
Same thing with the weather. Many Octobers are cooler, but there are years it's hot as, well...thing the Villain in Hercules.

Of course, I kinda like to try and be prepared as possible for everything when I go there, so nothing gets in the way of the fun.
Here's my opinion, for what it's worth... if I had just 1 day in the MK, especially on Halloween, I would probably eat at the quick dining places. It would also save a lot of $$. Most of them have lots of indoor air-conditioned seating where you could sit together as a family but take up a lot less of your time. There is Columbia House for Fish (and a couple of other things), Cosmic Ray's Starlight Cafe (where the Kooky animatronic alien lounge singer croons on stage while you eat lol), serving burgers ckn etc., Pecos Bill's, serving burgers, taco salads etc. and a hand full of others. That being said, I know some families that have a sit-down meal every single night when visiting WDW. That all being said, I know some families that have a sit-down meal every single night when visiting WDW. If this is your family, then by all means pick a table service restaurant. I might have to agree with you though, about not leaving the park. I'm a person who's often happy in the parks with a hot dog from Casey's or a burger, but that's not for everyone. Given the fact that you don't come too often, that you have 1 day in the MK and it's a special day, I'd probably not want to leave. However, since the MK and Contemporary are a quick hop from each other on the Monorail, that's not a bad restaurant to pick, if you are up for leaving the park.
You can find menus to most restaurants here. I am extremely picky so I find that very helpful. Hopefully no one will be bothered with me posting this link:
http://allears.net/menu/menus.htm
Yes, there are lockers in the MK. When you first walk in under where the train passes right up front there is a large area where you can get them. I'm really not sure how big they are, but if they're soft costumes that fold they'd probably fit.
It's cool that you will try to make your own costumes. I did that one year for Halloween & the Happy Haunts Ball (haunted mansion related special event during Halloween week that lasted only 3 years). I had no idea what I was doing, but it went ok and obviously no one else had my costumes.
I had been doing the dieting thing for months....well far longer than that, really. My vacation derailed me badly! I lose weight excruciatingly slow these days, but it goes on far faster than should be possible, going by the math of calories. I have SERIOUS work to do!
There is one other hint I'll offer, but I'd prefer to PM that.