You seem to be missing one key point.
Depressed people can get better.
People with the conditions which would meet the criteria don't.
I'm not suggesting it's not a real illness for a moment - far from it, just it's possible to fake depression, you can't fake paralysis, or medical conditions which cause intense pain (it's measurable) - which in this case isn't a risk worth taking.
Assisted suicide should not be done for people who during a temporary emotional state decide to go-to Dignitas to solve the problem - neither should people who are suffering what could be short term depression.
I've known many, including a family member who has tried to commit suicide & have failed, who now years later is happy & living a fulfilled life.
Because circumstances can change & depression can be cured.
Read what I said.
"is notoriously hard to diagnose fully" - it's not always possible to know 100% - this statement is true.
"& people are known to make full recovery" - this statement is also true.
It's pretty easy to tell if somebody is paralysed/locked in syndrome.
Don't try to insinuate that I don't believe depression or mental illnesses to be equal to more obvious illnesses - the brain is a also physical organ prone to error.
Going for assisted suicide for depression would be like going for any other illness which has a high cure rate - that's the point I was making.