Please look again at Doris Day's excellent answer. It's tough to read, but you need to listen to her advice.
As a mom who likes to do special things with my daughter, I really do know how much you enjoy that. But your daughter can't handle "special" because she can't even handle the day to day necessities (like a simple shopping stop that turns into a blowup hours down the road).
To Doris Day's spot-on assessment of the situation, I would add this: If you have to take her into any kind of store with you, never, ever go in without a written list. If she is a kid who does well with being given some responsibility, you can put her in charge of the list, carrying it and ticking off items as you get them. Never deviate from the list (even if you find something you just then realize you need; you're making a point here that you can flex on later when she's older, but right now the list must be the list and nothing that's not on the list goes home with you). If she cajoles and begs and fusses, you have the list: "If it's not on the list, we don't get it on this trip." Every single time. If she argues, ignore her.
And as others say, no more "you can have X if you behave." I know! You're thinking of it as a reward, not a bribe, and I get that, but she is only behaving (at least in the example you give) because of the promised reward/bribe, not because it will please mom if she behaves. If she's smart, and it sounds like she is, she might say, "Last time you promised me a slushy if I didn't ask for stuff. So do I get one today if I don't ask?" Be ready for this to happen and have your reply on hand: "No, not today. Coming to the store with me is a privilege. If you don't behave, you won't come to any store for a month." Make sure she knows what the definition of "behave" is -- does it mean not askiing for things that are not on the list? Etc.
I can see how the slushy-candy-dessert thing happened. We also rarely do desserts. You weren't thinking ahead when you offered that slushy, and dad might not have gotten the whole sweets thing when he offered M&Ms. You just need to think further ahead and to stop offering ANY reward that is food, period, because it sets up issues like that.
Have you ever tried the "I'm disappointed in your behavior" route? Tried to move her toward wanting to please you and dad for the sake of just getting your approval, rather than for the "rewards" she can get? Some kids respond to the idea that mom is disappointed in them, some are so stubborn they won't. But I'd try it with her.
You CAN gradually get her out of this stage if you and dad together are very consistent and stay calm. If she seems to enjoy arguing with you and getting you worked up, learn to walk away from her and tell her the conversation is now over; if you have trouble doing that now, you need to learn to do it. It will provide a "short, sharp shock" to her if she's used to your engaging with her and talking, then arguing with her -- Just go cold, say, "I've said no, and that's the end of the discussion. I'm going to go now." And walk off and do something else and don't let her get you to talk to her more, if she follows you etc.
Regarding the movie -- is your concern that she will see it as another bribe? Or that on the day of the movie, the movie stop itself will not be enough and she will demand candy, popcorn, and a shopping trip at the mall after the movie--? I would take her (as long as you have NOT set up the movie as some reward for something -- no more rewards like that!) but sit down face to face and say calmly, "We're going to the movie but because we are having dinner afterward, no one is getting anything to eat AT the movie theatre, and we have to leave the mall right after the movie to go to dinner. Got that?" Make sure she is looking you straight in the eye. Set expectations up front and walk swiftly by that candy counter when you get there. If you must, in the car as you arrive, repeat it: "OK, remember, no one gets any food at the theatre today. We are eating afterward."
Writing sentences as a consequence will mean nothing to her and just make her sit there and stew. Even teachers at school don't use that one any more. Did taking away things from her actually work to change her behavior when you've done it in the past? Then use it, but take care that she knows what you mean by good behavior -- at seven, she still needs specific examples from you of how "doing X will get toy Y taken away for one week" and so on -- be specific with the behavior, the consequence (make it something she really will hurt to miss) and how long the consequence lasts.