I just finished the RIV process (except I haven't licensed my car yet, just because I'm waiting to switch over my FL license to Ontario) so I can help. The 72 hour rule is a US rule. You must leave the car in the US for 72 hours then go to their export office at the border. They DO enforce this rule, so as inconvenient as it is, just do it. They WILL ask where the car is coming from. I made the mistake of ignoring the rule the first time and they wouldn't allow me to export it without leaving it there for 72 hours first. Total waste of a trip. I personally dumped my car over the border at the Niagara Falls casino parking lot for 3 days, LOL. The actual export process took only a couple minutes once she was satisfied I had the car in the US; she just checked that my car was the one I applied to export and sent me on my way.
It is a hassle. I spent $50 on the daytime running lights cheapo part (it was like $130 at Canadian Tire, I got it from an independent parts store) and my mechanic charged me $80 to install it. Other than that, I met all the requirements (restraint seats, French stickers, etc). But I still had to pass the safety and emissions tests which required a lot of work done on my car. Plus the import fees, the trips to the border... I've spent a good $1500 on the process at least so far, and my car is only worth $2000.... x_x If your car is old and not great like mine, you'd be better off selling it.