I think your missing the point here that it wasn't an autocross event, it was an autocross school. I have attend one CVR-PCA school and one Cart-Ct school and in both cases you had to be preregistered because there was one instructor assigned to each student, one on one. They wouldn't allow anyone not preregistered because there wouldn't be an instructor for that student. From my experience DE's are run exactly the same way.

Autocross events are different because they are timed events and you are competing. Show up, sign up, race.

As far as miscommunications between you and NER-PCA, That is a different issue and all I can say is, some of my communications with my local PCA chapter have been, at times, very frustrating and I felt like I was being ignored. I was pleasantly surprised this week to get a prompt response from the CVR-PCA regarding their autocross season(not good). SCCA might be a good alternative.