We need a current Sarge to weigh in. I'm wondering because I was reading one of the white papers or something where a newly promoted sergeant had asked to act-up with a more experience sergeant for a month and the request was denied and he was thrown in at the deep in. He survived but his complaint was that he would have been more effective in his role with more otj experience and guidance.

I think this is not a crime that you listen music in your car. But if there is any law regarding this then you should obey law to avoid any inconvenience. So confirm this from your law department.

If he has injested the substance prior to police arrival we cant prove possession so that is out the window and to arrest him on suspicion of possesion doesnt solve your problem.

I would personally have traced a next of kin or appropriate person and released him into their care. We have a duty of care for people up to a certain point and when we can discharge that duty responsibly we should as a police officer is a valuable resource and not one best used babysitting someone at A&E for 5 hours.