Just a thought...could you move the calves to ground that hasn't been used by the cows...or any other calves, for that matter. All kinds of nasty bacteria can live in soil. We have a place here at our farm that I wouldn't put a calf on now if my life depended on it...'cause they'll scour and die in a heartbeat. Also, my brother, who's been a dairyman for most of his 52 years, as my father was before him, says that if the calf can't keep the cow milked down, you need to milk the cow. Sometimes their udders get so swollen that the calf can't get it in its mouth and effectively suck it and if the cows aren't milked down they can wind up will all kinds of teat problems.
Myself, for the scours, I'd give them an antibiotic that's a little more powerful than scour pills. And keep the electrolytes going. I know that mother's milk is always better, but if you're really worried about the milk they are getting, maybe you should switch them to a high protein milk replacer and feed them small amounts throughout the day interchanging with the electrolytes. Oh, one other thing...microbials...push the microbials (Pro Bios).
I feel for ya'...
Alice