Former Census CSU worker here.

It is mandatory to fill out the census. Takes us about 3-5 minutes to help someone fill out the short form over the phone. 15-30 minutes for the long form over the phone. A completed census form is required for every household under the statistics act. Failure to comply may result in a fine and/or jail time.

All the information collected is kept confidential. Every statistics Canada employee is sworn to secrecy for life. Breaking that trust holds a much more severe punishment than failure to complete the form. The information is not shared with any other government organisation but rather used to create statistics with which other organisations can best plan how to serve and allocate resources to the provinces, cities and neighbourhoods. Also helps the government create policies. There are inumerable applications.

No, you can't ask for a short form if you have been giving a long form and vice versa. To allow this would cause respondent bias, which was the problem when the long form was not mandatory.