Far from a language expert, but Wiktionary does list the character as having both a saki and zaki reading when part of a compound, so Onisaki does seem to be a possible. The reason for the Onizaki reading default might be due that the Zaki reading being more common for this, for example: baseball players Onizaki Yuji and Onizaki Tomochika, the former town of Onizaki, Onizaki fishery cooperative association, etc.
Far from a language expert, but Wiktionary does list the character as having both a saki and zaki reading when part of a compound, so Onisaki does seem to be a possible. The reason for the Onizaki reading default might be due that the Zaki reading being more common for this, for example: baseball players Onizaki Yuji and Onizaki Tomochika, the former town of Onizaki, Onizaki fishery cooperative association, etc.
Yeah, that's what I suspected. More accurately saki undergoes rendaku and becomes zaki half of the time. However, as understandable as the confusion was, it still means we've been using the wrong reading of her name. I.e. different than the authors intended and the official sources support this. You could try to find an official voice line of her name from the game to make extra sure though.
You could try to find an official voice line of her name from the game to make extra sure though.
A video of her character introducing herself along with announcing being a playable character in Action Taimanin. YouTube link. She says her name at 1:09 and again at 1:30.