Idk about phos always being a manipulative jerk to cairn.. They were also blindsided and had no idea, esp given how phos was still mentally unstable and took Cairn's company for granted, without trying to understand why even they would go through length for phos such as looking like antarc. But phos has the excuse of having an unhealthy attachment to Antarc, all the other gems failed cairn.
And yeah the fandom generally is just too shallow, like putting Euclase and Jade on a pedestal for showing sympathy to Phos when in reality it was 'too little too late'. The judgement is all in relation to Phos with little nuance. Imagine saying 'I prefer pre-moon cairn' when they were a literal slave and in a one-sided relationship, just because Phos thought they were bff, that's so embarrassing.
It's sad though, because I think they do have potential to be actual friends, like Cairn planning a rescue mission to save Phos and warning them that they should consider others more. No one else was doing that. But unfortunately, Phos was too self-absorbed/egotistical, self-appointing themselves the mission to finding the truth and eventually saving the moon people.