Any bishop who determines that people MUST NOT kneel prior to recieving communion has a serious pastoral problem. Considering such reverence for the Lord as a mortal sin leaves no doubt as to the priest's pastoral problem.
I am quite certain that any priest who would publicly proclaim that kneeling is a mortal sin is the same sort of priest who would at the same time refuse to proclaim that abortion or deliberate defiance of the Church's teaching on birth control is a mortal sin.
Kneeling a mortal sin??? That is one troubled priest (on a power trip) who needs to be prayed for