Mmm, actually, most of them I can't really answer, as he doesn't really delve on those aspects
. He says he's not the open-world type and that his exploring the world is different than usual (he would complete some side-quests, but he was mostly about sight-seeing), so he doesn't give much away about NPCs, apart from noting who the stronges outside of the four is and that supporting characters add flavour and colour to the world (and this is, first and foremost, a Noctis game). Nothing on Niflheim's bosses .
Actually, he doesn't talk much about plot at all, so we don't know about Regis, the antagonist and all that. Regarding the summons, again, nothing, but he does have a section in his review to talk about set pieces and how they're, for him, what FF is all about (like Sector 7 in VII, the Gardens' fight in VIII...) and how they're very well implemented within the combat system (whether that means more battles against Summons beyond the ones already seen, I don't know). Also, nothing on collectables.
Sorry, I realize that this was a very barren post, lol.