Regarding the first point, simply put, the Canadian government feels it is in its national interest to not accept being part of a foreign electoral constituency. That is its policy. Other governments have their own policies. I would add that this issue is certainly different in a European context, for instance, where integration is very strong, and probably in the American context, where identity is perhaps different. However, I want to emphasize the fact that this is truly a Canadian policy.
As for Tunisia's specific situation, I do not want to get into any details. Let's just say that the constituent assembly we are talking about would have rather considerable powers, such as drawing up a constitution and probably passing legislation. That is very similar to the powers vested in the members of a national assembly.