I missed this plot hole (or at least perceived plot hole) on the first reading, but I realized that there may be an issue in that as Astoria is a huge Malfoy fan at this point, she might not be willing to swear not to do any harm to the Potter or Black families. This may or may not be something you'll want to address along the way. Still a great story and read. thanks for your hard work on it.
Author's response
Hmmm, interesting idea. Normally I'd say that since she got the portkey from her folks who did swear that she wouldn't have any trouble. However I suppose if I wanted something to occur I could say that magic recognized that it was her who used it and/or that her father's oath covered her as a minor. I thank you for the idea whether I use it or not. As Artie Johnson used to say, "Verrry Interesting!"