Shanh

I'm troubled by the mean-spiritedness of some of the comments. Where's the compassion? Perhaps her English is very basic, resulting in messages being lost in translation. I doubt she is planning to lead a life of crime here.
Although the sad truth is that K1 must either lead to marriage or a return to one's country, I truly wish her the best. She came here to be with her fiance, and that wasn't easy to do. Now she has to deal with his cold feet and what looks to be the breakup of her relationship. She's looking at a long flight back home with a broken heart. Why pile on?

It would have been best to file with your married name, regardless on how it is in BE, but now that it's done it's a matter of costs I would say.
Waiting to file for I751 would probably be less costly.
As for Belgium, there will be no confusion, you need to register to the consulate etc and they will take it from there, all in your maiden name.
Only 2 things that have changed as far as Belgium is concerned is your residency and your marital status.

A copy of the marriage certificate can be obtained in the state where the marriage took place.
They keep record of that

Well maybe since it was only since yesterday she is having an emotional reaction to not being able to get her dream wedding. I know people say a lot of things they don't mean when they're upset over something. We can hope at least! I'd say bring up the alternatives for $3500 and see if she'd go for that. You CAN have a beautiful wedding and spend less. We only spent $300, went to Lake Tahoe and it was lovely! Very small but I liked it that way, not too fancy but memorable enough.
If she won't go for that, then I would seriously consider if you should stay with her or not, I mean that in no offense, but if she's being selfish enough to postpone and blow thousands of dollars just to get her way, it's not really worth it. You're both young and need to think about saving up for a future, not just a wedding.