2) If anything missing, suggest on our roadmap...we can improve https://github.com/dayschedule/public-roadmap/issues
3) The development repo is private and not open-source
4) You can create multiple pages on dayschedule.com and use popup option to add multiple buttons to display as many calendars you want. See this example in HTML https://stackblitz.com/edit/popup-appointment-widget-for-web...
5) See details on website https://dayschedule.com/
6) Checkout my link https://vikash.dayschedule.com/ (The same is embed in WordPress)