3. Is the cb_asignmessageuser1 field the correct field name to use or is there a typo and it should be cb_assignmessageuser1?
It's just a typo, but it's also just a guide so you can name the fields whatever you want.
4. The section starting with "CB Fields on Accept Family Member tab"... Is the "Accept Family Member" tab another one that wasn't listed in the first section of CB Tabs to create or is it one of the tabs already listed?
It's a tab you need to create.
5. I just got to the part about creating the "Family Plan" itself. I nave 3 different subscription types all with different pricing and amount of extra users each one can have. Many of the fields that are to be set up through this tutorial look like they are being set for just one plan. If I have 3 plans do I need to do this setup 3 times with unique plan names?
It's designed to share a single subscription. As we don't have subscription sharing functionality built in for built in family usage this is just one crude solution to try and force it. Using generated promotion codes is probably significantly more flexible.