Not sure if I can help, but I have 2 cats in the basement who refuse to come upstairs. Their only link to the outside world is Antonio. He will go there and hang out with them, but they refuse to acknowledge me in any form...I've had them for over 4 years now...they will come close enough for me to touch their nose but that's it....I guess some cats just prefer other cats.
As for the nursing, was Mia removed from her mom too early? She isn;t getting any milk, so I would guess its just a soothing/grooming ritual that they both enjoy. If there is no harm, I guess it should be ok.