I don't have definitive answers to your questions, but I'd make a couple of recommendation about things to keep in mind while looking for your answers.
First, remember that coverage is determined by the plan. The plan that the patient is enrolled with decides what they do and do not want to include in coverage. Unless coverage of marijuana, or the services related to the prescription and management of it, is actually prohibited by law, the coverage will be based on the individual plan requirements, so this is likely to vary from patient to patient and you'll need to look at your patients' payer and plan guidelines to get a real understanding of this.
Second, I would say you need to consider that coverage of marijuana as a drug would fall under prescription drug benefits, whereas coverage of visits for the treatment of an illness will fall under a medical benefit, so these should be kept separate. So I don't think it's correct to say that a visit has to be automatically considered non-covered simply because a provider may prescribe or recommend this as a treatment option. Providers often recommend medications or remedies that fall outside of coverage (e.g. vitamins, massage, acupuncture), and just because those things aren't covered doesn't mean that the visit is also not covered. The coverage of the visit should be based on the service provided and whether or not that service meets the definitions of coverage in the patient's plan. For example, Medicare defines a covered service as "services or supplies that are needed to diagnose or treat your medical condition and that meet accepted standards of medical practice." So by my interpretation, a physician visit to evaluate and treat a medical condition would not necessarily be excluded from coverage simply because the physician recommended this particular treatment option.
Finally, since these are really legal questions that may require a more detailed knowledge of both state and federal law, if this is going to impact your practice in a significant way, your best option may be to seek the advice of an attorney rather than an internet forum to ensure your procedures are solid and compliant. Hope this helps some.