It's no coincidence Mannering has been absent for all but one test in that period, so why would you consider leaving out the country's best defender for the World Cup?
The second point is more subtle. The Kiwis are a confidence team, with many players who find their belief and best form when things are clicking on the park. Mannering is one of the few, such as Thomas Leuluai, Tohu Harris, Roger Tuivasa-Sheck and the soon to be missed Jesse Bromwich, that sits outside that group. He plays well regardless of circumstances, thrives when the going gets tough and has the ability to bring the best out in players around him.
He's coming off another solid year with the Warriors, where he added some offensive spark to his game as well as doing the defensive work of three players each week. Sure he made a few mistakes, and maybe missed a tackle or two, but Mannering also made more tackles than almost anyone else in the NRL (the second most with 986) and was one of a handful of Warriors who emerged with credit almost every week.
Thirdly, he's a leader in a Kiwis team that doesn't have many. Bromwich is absent, Tuivasa-Sheck and Shaun Johnson are growing into senior roles, Adam Blair has limited captaincy experience and Kieran Foran has other things on his plate.
For the selectors, clarity should also come with a simple rhetorical question: if the Kiwis were leading with 10 minutes to play in the World Cup final, would you want Mannering on the field, or sitting at home in Auckland?
There's nothing wrong with good constructive debate, which will lead to the best possible outcomes. But it feels there has been some over-thinking in this case and if they are really considering axing Mannering, the panel has lost sight of league reality.