In terms of the events entered, they are two separate events. Two organisers, two entry forms. There is a rule about the time limits, STFW for it. I don't know what it is. Not a question of what I would allow, I don't make the rules, just expressing my view on my interpretation of them.
I actually think it's important to log that the calendar was finished, the extension plus the calendar DOES give the total distance.
Yes, you're right about two entry forms. I agree that the calandar event should be logged as being completed too.
Perhaps the event could be done with just one entry form?* The rider would still need permission of the calander event organiser to ride the event, as well as the brevet card and routesheet, so would still need to enter the event because the brevet card would act as proof of passage.
It would also mean that failure to complete one of the rides, either the permanent or calander section would be a failure of the event, which was entered under one entry form.
It is a tricky question to answer though.
*I think that's what we did with DIYs which had calander events as a section of the ride.