Within my Google Calendar I have several different calendars set up. Is it possible for someone else to send invitations directly to a specific calendar (not the default)?
I doubt that something like this is currently possible with Google Calendars, and mainly because that functionality is not supported in the iCalendar format. What is possible is that you may move events between calendars in your account by editing the calendar details and choosing an different one from the drop down menu
Here is a link to a similar StackOverflow question, How to post calendar event to participant's specific calendar