I think a MH assessment beforehand will only indicate so much, it's a really unnatural and unique environment and without thrusting people into it any checks they do really are best efforts etc. I can see why Niall feels aggrieved because the show wasn't what he thought it would be and I sincerely hope that LI producers have addressed this in the series since, but they sought help for him when he needed it, didn't betray his confidence by saying why he had left and let him control that narrative for himself. Short of turning back time not sure what more they could have done, he's just coming across as bitter now and should move on rather than clinging onto his 5 mins of fame whilst conversely be moaning about it.
I do agree that a 25+ cast would be better though, if nothing else it'd be more interesting I expect. It's hard to tell with Luca and Gemma as we only see one narrative which is the producers; many ex islanders have said how compatible they are etc and we don't seem to see that but it probably is the case as no other reason really why people would lie surely.
I work with young people who are experiencing, or have just experienced, a psychotic episode and I think Niall's openness about what happened to him and how it still affects him now is actually really helpful. Psychosis isn't really talked about much, especially on platforms that specifically engage younger people, so I think it's great that he's spreading awareness and making it ok for people to discuss it and ask questions.
I can see how he might come across as bitter towards the show, but tbh I would too in his position. Plus, he talks about the show as part of reflecting on what triggered the psychotic episode, and that's actually an important part of recovery- you need to identify your triggers so that you can reduce the risk of it happening again. Again, I think it's good that young people aree seeing someone processing their experiences without fear or shame.
Sorry for the ramble, destigmatising Psychosis is something I'm very passionate about.