She may have been given better gear, but her combat experience is still close to 0. Why are you expecting her to suddenly beat a bunch of experienced combatants? People other than Diablo do lots of stuff. Much of it isn't related to fighting, but that doesn't mean that it's not important...
Well, they had a cake, noisemakers, and then the Happy Birthday message just like they said. I guess it just goes to show even if you know what will happen in general you can still be surprised by the details.
It's not really intended to be a wish fulfillment story. One of the first things the author puts in the forward of the original version is that the MC's kind of stupid so she generally won't make the best decisions and there will sometimes be lasting consequences from it.
It's not that anyone is getting married. It's just that as her brother he's discouraging her from doing things (like making weird faces) that he thinks might discourage potential suitors.
Is is, of course, possible that Maou has no intention of upholding the bargain once they get back to a world where their powers work. They didn't hesitate to trick the angels before, after all.