Not sure what the benefit is of allowing a conflict flag parameter on the ATTENDEE property. This seems to introduce a level of complexity that may be difficult to show interoperability for. I think you would agree that we need to nip-in-the-bud any extensions that will negatively impact IOP.
I can see the scenarios for having a modal, CAP specific, calendar level property that says "check and notify me of any conflicts" on events that I create during this session. But I see only problems with a modal, calendar component level property that is suppose to restrict creation of conflicting components. If the property is set of TRUE and you create some properties that are overlapping and then you reset the property to FALSE, is the calendar system suppose to do anything about the existing overlapping events that would otherwise not be allowed with the FALSE value? If not, then I think that this property has limited utility.
Better to have a CAP property that merely is active for the session and will notify a CUA if they try to create an event that overlaps an existing event.