USMD here going through CARMS and NRMP (feel free to message me questions, cause its a pain to navigate on your own)
1) Dont trust the stats as people have said many applicants only want specific schools so their CARMS ROL may only be 2 long!
2) You have to make sure you tell both systems you are registered in the other and they will make sure you only get 1 match
3) Many USMDs apply to family but I know an equal number who didn't and still match in CARMS (I even know a few rads), just make sure you are doing electives in Canada. Electives don't guarantee you interviews at that school but you need strong Canadian letters to get interviews (these tend to be regional Ontario letters = Ontario and Atlantic interviews, Man, AB, Sask and UBC, tend to get interviews in west of Ontario. N.B. the sample size on this is small). I also know some pediatric residents from well known US schools who did no electives in Canada.