Nobody is trying to diagnose your daughter OP. People were just pointing out that, actually, they wouldn't rule out ND as what you have described sounds like she could be. We can only go on the information you've given us, which would point towards there being a possibility (nobody here has said she IS ND, just that she could be). You're right, we don't know your daughter, we're just pointing out that you may have misunderstood ND by automatically ruling it out.
You ask what benefit a 'label' would have. Well, it would help her to understand herself better and for others to understand her. It would help you to learn strategies to deal with meltdowns and pre-empt them to reduce the risk. It will hopefully make life a little bit easier, if she is ND, to have a recognised diagnosis.
Yes, she could indeed just be a brat. In my experience though, children who are just 'spoilt brats' are less likely to mask in different situations. That's their personality and they're generally like that everywhere. People often have this idea that Autism can't be controlled so therefore children who mask can't actually be autistic but autism presents in so many different ways. Children with high-functioning autism often sense they are different. They desperately want to please in school, and spend all day desperately trying to follow the social norms they don't truly understand, putting on an act for fear of rejection if people at school see the real them. They are choosy over who sees their meltdowns and their quirks. They relax at home, and that's when the meltdowns flow as they finally release the stress of the day. Look up the 'coke-bottle effect'.
If you don't feel this fits your daughter fair enough, you do know her best. People aren't trying to suggest you don't know her, or diagnose her based on small snippets of information. People are just trying to help you to help her by pointing out that it could be a possibility. You didn't say earlier that you had researched ND, you just appeared to dismiss it. All people are doing is pointing out that it's worth looking into if you hasn't done so already. Ultimately people's suggestions of ND is to try and help you both, if it is a possibility she is.