Ok nevermind what I said last month.
Hitomi Sato is definitely involved (that explains why she retweeted stuff about it). The last theme heard in this video and also in the Pokémon Presents one could be another one of her songs.
Ichinose is most likely involved too with
this new "Battle! (Trainer)" arrangement.
Still have no clue about who made the live themes mentionned earlier though ("Lumiose City", "Battle! (Wild Pokémon)", and the 1st "Battle! (Trainer)" arrangement).