First of all, don’t use GetNthDocument(). It is SLOW.
I am not really clear on if you are trying to set the choices for the combobox or set the actual value. A combo box can not contain multiple values, you need to use a different kind of field for that, like a listbox.
If you want to set the different values that the user can choose from in the combo box (the options), you should use a separate multi-value text field, populate that with the values and then use a formula in the choices section of the combobox to get the values there.
Since the code is executing in PostOpen, you should update the uidoc, not the backend doc. That could be one reason it does not work.
You never declare/set the variable “item” to anything. It is also recommended not to use the extended syntax when getting values from documents, use GetItemValue() instead.