How can I make custom (participant attendees) reports with inline editing functionality?
To clarify - the nz.co.fuzion.extendedreport extension offers Editable Reports for Activities and Events - but not for Participants.
Eg with the Events report you get fields like this

Another option would be to try Drupal Views and Views Bulk Operations. I am pretty sure we have made it work for an 'event register' ie have a list of folk who are Invitees, and then tick those who show up and VBO will change their Attendee Status (don't recall if we added the necessary so that the Status is a dropdown or you have to enter the status ID)