If someone owing you money gives you a few items of value to hold onto until they get the money for you, is there any risk of them reporting it stolen to the police?

Yes, there's a risk they will report the items stolen.

Will the person in possession of the articles be arrested? Not if the Police get/understand/believe the version you are telling (which I have no reason not to believe).

Of course, if they DO report the items stolen and they are NOT stolen there's the matter of filing a false Police Report.