Do we know for sure that Aversa is going to be the Grand Hero Battle on the 25th?

Yes, there was an announcement on twitter.

Yup! we do, you can see her in the trailer. Also.... https://twitter.com/FEHeroes_News/status/1054209959481634817

Theirs no other new banner's coming out other than units with Smoke and Drive skill's and the L.Banner but they wouldn't show that unit so early, Aversa will come out be4 the new Forging Bonds event on the 27th and the only other thing going on is the GHB