The examples all show only one entry in the List.  If you're going to have a List resource, the List must always include all entries.  You can't query a list and have it only show the one entry you were searching on, it must include all entries that are part of that List (or at least all entries the recipient is allowed to see - and what they're allowed to see can't vary based on what search parameters they specified).

Also, the MedicationKnowledge resources have references to the List, which means that there's a bi-directional mapping - which is also very bad practice in FHIR.  In general, references between resources should only ever point in one direction (many to 1, new to old).  From both of these perspectives, having the drugs point to the plan makes sense.  However, the reference is being done by id string rather than by URL, which means that the linkage can't be resolved.

If you don't want to pass around whole formularies, it seems that you should be using InsurancePlan as the resource for the Plan and have the MedicationKnowledge resources point to that.  Using List as you're doing it doesn't really make sense and isn't adhering to the semantics or behavior of the resource.