i've seen some travel post about a hk ppl travel from london to hk by train, he has hksar passport, hkid, and 回鄉卡. no problem when entering from mongolia to china with 回鄉卡, 回鄉卡 is for hk/macau ppl to enter/exit mainland china no matter to/from which place
make sure you line up at the 中國公民 line at checkpoints
also, if you apply china visa with canadian passport, you can't renew your回鄉卡 when you back hk