There is no inability for you to live common law. You are choosing not to for financial reasons, as you stated above. Lots of us quit our jobs and waited out the PR process on visitor visas. Living on one income really sucks, sure, but it's not an immigration barrier, not to mention in CIC's eyes there is nothing keeping you from getting married. It'd be different if she had to apply for visa to visit Canada and was denied, and you applied for a visa in her country and were denied. In CIC's eyes, you are choosing not to get married, and choosing not to become common law. That is NOT an immigration barrier.
I respect that you both were at University, but CIC might raise an eyebrow over the fact that you've been together in the same country for 5 years but never lived together. That certainly won't make your application easier. Americans do have lower burdens of proof for genuineness than other countries, however. But to apply for something like Conjugal would be a waste of $1040 + medical exams + all your time and energy.
Moving to another country for someone is a big, big commitment. It's not to be taken lightly, and it's a lot to give up your career, your credit, move away from your friends and family and start over in a new country. In my opinion getting married looked like signing a piece of paper in comparison.
I hope you both can talk and decide where you want your relationship to go in the future

She could always come up on a study visa, or a work visa, but you have to be pretty specialized to find an employer that is willing to pay for a LMIA.