Ex assessor here just chipping in on a couple of the questions asked.
One reason for the ammount of decisions that are overturned is that assessors are told they cant score over a certain % of claimants, if there numbers get too high they get put under review and are monitored in the assessments and put under a lot of scrutiny.
Assessments are also sent to auditors who can be even stricter than we are and tell us to drop scores down as they don't believe theres sufficient evidence. Therefore we have to change it even if we don't agree with it, so even if it's not the assessors decision it still has the assessors name associated with it.
Another reason is that the assessments follow a very rigid criteria, and these criteria's have tightened throughout the months/years. One example being for the activity 'dressing', if someone is able to move their hip to a certain angle (very small) bend over to their knee or bring their knee at least someway to their chest then that is seen as adequate to dress yourself, very few people are unable to complete these movements as the degree of movement is so small. Another example is 'following journeys', unless you present yourself in assessment as vomiting, screaming and crying in distress you wont get anything for mental health.
The person asking about qualification. Its expected that someone has 2 years working experience as a health care professional, e.g nurse, paramedic.
And in regards to the query about expertise in the relevant area, it doesn't make a difference in the assessments, as mentioned before they follow a criteria. To score points it's about the evidence available. So evidence can either be; how you present in the assessment, the medical input you have, the medications you are on and information about your daily life (e.g giving up work, having home adaptations etc). So for example you can have a physical health nurse assessing a young man with adhd as long as he is getting specialist input for adhd, is on medication for ADHD and presented as you may expect someone with ADHD to present, and hes attending a specialist subsection at his school. That would be adequate information to score despite having no prior knowledge to score.
However that is where the major flaws lie as theres many reasons someone might not be on medications, or have specialist support, or might not look unwell at the time but Pip has no considerations for these reasons and is very black and white.
These are also some of the many reasons I'm an ex assessor.