HaLeivi,
I’m not sure why you stetch the definition of kefirah in this case, yet in other cases, the first word out of someone’s mouth is labeled as kefirah.
As for Machnisei Rachamaim, my understanding is that it’s 100% kefira and everyone knows it, but since we say it, it’s suddenly allowed. That still doesn’t take away the fact that it’s a prayer to angels.