RLS is generally diagnosed based on symptoms, PLMD by a sleep study, for which your father is scheduled. Clonazepam is sometimes prescribed, but he has already experienced the side effects, unless a lower dose would help his legs without such strong side effects.
There are some over-the-counter meds that claim to help calm the legs and help sleep. They usually contain magnesium and some other supplements. Another thing he could do while waiting for the sleep study and results is ask his doctor to order a Ferritin level, a blood test to check storage iron. My doctor wants my Ferritin to stay above 60 or even higher. Also, it wouldn't hurt for them to check his kidney finction.
If your father indeed has a severe case of RLS and/or PLMD, it truly is a miserable malady. RLS usually does not require a sleep study to treat, as you know since he was already given a med for it. Maybe the same doctor would try one of the dopamine agonists for it. Just be very wise in their use. He should start off low and increase gradually, staying at a dose long enough to give it time to work. He does not want to use any more than necessary due to possible side effects, which the family should be aware of and observant for. If they give him these meds, he should NEVER stop them abruptly. Wean off at a recommended rate.
Hope your father gets some relief soon.
I kept a note of what you have written and will pass it on to my father.