You cannot realistically get one without the other. Treatment often cannot be cold turkey, which means the addict will have to keep consuming (and hence carrying and buying) for a period; if that's illegal, treatment is not going to work. And no, methadone is not the same.
Taking Sweden as control, for example, they spend loads on health services and treatment; but without decriminalisation, their numbers are still terrible.
Sure you can. Restrict illegality to use/possession outside of a treatment program. There already are many things pharmacists are not allowed to sell without a doctor's prescription, including strong opoids.
You would probably not want this kind of prescription to be signable by just any MD though, but that's another story.
Taking Sweden as control, for example, they spend loads on health services and treatment; but without decriminalisation, their numbers are still terrible.