Hi. I have been meaning to do a post on so called Competent Authority Services office of the CRA in Ottawa. I do suspect there are some limitations in what the CRA can do under the treaty; however, I suspect for those that have renounced (such as Petros) or have lapsed Green Cards I suspect this office can be of some assistance. The CAS office is technically a free service of the CRA; however, it has a reputation for only providing services to the very wealthy and large corporations (something I think is due to the fact they are the only ones that know of its existence). I have linked to several information sheets the CRA has published regarding these services below. The key document to read is CRA information circular IC71-17R5.
I also linked below to the annual report of CRA CAS below that shows a staff directory of the people who work in this department in Ottawa.
The other key thing to note since 2007 is the US and Canada have a binding arbitration mechanism in their tax treaty. (Historically the US hasn’t liked binding arbitration and Canada is the only country the US has such a mechanism with).