Donohoe claims to have a cognitive impairment. Which is said to affect memory, learning new things, concentrating, or making decisions that affect everyday life. He seems to have had a disability support person in the silent police interview, so I guess it is a documented impairment.
Like you, I think there must be more to his inquest involvement than being a creepy adult sex offender who sometimes slept in his van nearby. The police were able to obtain a search warrant on his personal possessions.