An observation mission would be excellent and would address your last question about what the Nigerian government is saying.
The Nigerian government can say one thing, but what they do is another thing, so an observation mission is an excellent idea.
In addition to that, what I would suggest is to impose sanctions. It could be easily done at the international level, and by Canada even, because Canada does have some trade relationships with Nigeria. Imposing sanctions could have a great impact, in my opinion—