I got randomly selected for the long one in 2010. There were several questions I was not able to answer but providing incorrect information is a crime and not answering is a crime. So I did nothing. They sent one or two threatening follow ups. Then an in-person enumerator showed up and ran me through the short-form questions only, which was fine.
> providing incorrect information is a crime and not answering is a crime. So I did nothing. The correct answer in this case is to say "I don't know". You can write in stuff on census forms and it's up to the local office to process those as special cases.
In my case the correct answer was to do nothing, then cooperate with the enumerator on the short form questions, which as I said was fine.