PD sent back answers to my questions about races and nationalities (see http://hcmc.uvic.ca/blogs/index.php?blog=22&p=7911&more=1&c=1&tb=1&pb=1 ). To summarize:
Multiple races/nationalities: there should only be one value per census record for race and nationality, as per the 1911 Manual for Enumerators. PD will send a corrected spreadsheet.
Infirmities: though rare, multiple infirmities do occur. The fields aren't simple booleans, either; the enumerators were instructed to record the age at which a person's infirmity occured (some enumerators didn't always follow the instructions). So, we are going to implement option #3, a lookup table for census record -> has many infirmities relationship.