Less desirable graduate school vs. more undergrad
My reading of your question is that you already know your answer. You don't seem to be time or resource limited (as many others would be) and you express pretty strong preference for a second undergrad degree. 

The only caveat I would have is whether you are feeling inadequate when it isn't called for (Imposter Syndrome). 

An undergrad math degree (outside the US, at least) might not require you to repeat a lot of things as it is probably focused on just the Maths. So it wouldn't be backing up, really. Actually I don't know how similar the Canadian Educational System is to the US. Down here an UG degree requires a lot of things outside the major (Language, history, ....). Other places an undergrad degree is more specialized on one topic. Doing it in a year or so would depend on the university hosting it. Your original institution might be the best place for that. 

Also, rusty is bad. You will want to somehow bring your work process and thinking process back up to speed. Any sort of study can help with that, but a grad program is typically more intense.