Count rows in lookup table based on specific values missing in fact table

(Updated this as my sample matrix was incorrect)

Hi, 

I am still learning DAX and hope someone can help with this. I am looking for a way to create a measure that counts the number of entries in a lookup table that do NOT have a corresponding entry in a related fact table that matches specific criteria (based on a third, unrelated reference/slicer table). 

As a much simplified illustration, let's say this is my lookup table:

Registration Table:

And this is my fact table: 

Attendance Table:

My third unrelated table will be used as matrix column headers for the sessions:

Sessions Table:

In my matrix, I would like to get a result something like this (Note: Column headers are drawn from the Slicer table.):

Normally, I would simply subtract the number of attendees for each session from the total registrations. However, in this case, I will later need to apply specific criteria to the registrations which are missing records (e.g. to differentiate between isolated absences and full withdrawals), so I need to somehow POSITIVELY identify the missing records by session and registration. Is there a measure that could be used to identify these "missing" records?

Thanks.