I’m going to get additional advice on this, just to make sure I’m not being annoying for no good reason.
Yep, I’m right. Your original post says it’s to be a combination between those two stories; you don’t say inspired by. A combination suggests the same characters, settings, etc, and for that you’ll definitely need permission first. You might be thinking “Yeah but look at all the Harry Potter fan fiction out there? What about all that?” That’s because she’s explicitly said she’s okay with it. Just as long as no one’s making money off her copyrighted material, that is. Otherwise, she’d be well within her rights to demand sites remove them, as some other authors have done.