Is nuvigil effective for idiopathic hypersomnia?

Yes. Nuvigil is a central nervous system stimulant and can be effective for hypersomnia (sleepiness) of any origin including idiopathic hypersomnia. The response to Nuvigil is different in different individuals.
Often, yes. Nuvigil can be used to diminish excessive daytime sleepiness related to ideopathic daytime sleepiness. The degree of effectiveness will require a trial through your physician.
Yes. Provided all the causes of hypersomnia, in particular sleep disorders have been ruled out.