They do not do a 5 minute assessment
Yes, I know, and have never suggested otherwise.
It’s an in-depth assessment done by a team, usually 2 doctors and somebody like a social worker
This depends on where you are. Where I am, it's an MHO, who is not a psychiatrist, who does the bulk of the legwork, including usually trying to cajole you willingly to hospital, whereupon they'll assess you for hours if that's what it takes, before the duty RMO will usually consult with the MHO, perhaps ask a few cursory questions, and then if they agree it's necessary you'll be detained on the authority of the RMO, the actual duty psychiatrist.
Said child had a long complex history as many do when it happens. The team communicated with professionals already known and the assessment was long. It was not a 5 minute meeting done with zero knowledge
This is your child. One single example of a detention. What do you suppose happens when they pull someone off the street who has had no prior involvement with MHAS whatsoever? It's exactly the same process, and yes, the RMO is expected to make a decision about someone they've never met before and likely have precious little background information on, hence why they usually defer to the MHO. I've sat through countless MHAS assessments and I can't recall a single instance of an RMO refusing to detain someone if the MHO has suggested it's necessary.