I’m in the same situation: Main site calendar have to show his event and all the event of the subsite, the subsite has to show only some events of the main calendar.
I’m doing like that:
subsite setting: all the event are in the central calendar and the admin can only post in the central calendar
category: I created some category like subsitename1+typeofevent1, subsitename1+typeofevent2 and i assing the category to the event i want to be shown in the subsite1 calendar
in the calendars using shortcode or widget:
in the mail site show all the events
in the subsite1, show only the category subsitename1+typeofevent1 and subsitename1+typeofevent2
It’s work for me!