In Muvi Standard Plan you pay US $299 per month and in Professional & above you pay US $499 a month for Roku TV App. See our Pricing
All you need to do is purchase Muvi One + Roku TV App subscription. Fill in a small form available on your Muvi One CMS. Muvi team takes a week for building your Roku TV App. The app will be released to you for testing and confirmation. Once confirmed your Roku will be sent for Approval from Roku which takes about 2 weeks to review the app. For more details see our Help Article
Yes, Muvi One Supports all types of App customizations within the guidelines laid out by Roku.