Why didn't CWD appear before 1967? It should have wiped out all susceptible deer before the white man even arrived on this continent.... Oh, wait.....
I think you are overstating the virulence of it (my fault, really, as I've been keeping it vague in many of the updates) That said, I will accept that the "environmental reservoir" of RRS is the shakiest part of the disease- just as it is the least understood method of transmission in CWD. However, I think that the range of the Chisholm Trail and other cattle trails would offer a much broader chance of transmission than the more limited ranges of deer and CWD (as well as the greater appetites and... excretory functions of Longhorns). That is mere speculation on my part, and I will accept that it may be a bit implausible.
Now, as for RRS's transmission through milk, I took that from a study done on prion diseases in rats and its speculations about BSE/CWD transmission (for which I could not find any studies specifically on milk transmission). It was found that the irritation of the mammaries during nursing actually led to prions becoming concentrated there and being spread through the milk. This provides another means for RRS to spread without the "environmental reservoir".
In the first update, I did use a bit of artistic license. I imagine that the onset of the disease in a herd would indeed, frighten even the healthier members of the herd, especially among an ill-disciplined breed such as the Texas Longhorn. Thus, it looks worse than it actually was at Red River Crossing. And again, as a "spectral ailment", it will have many exaggerations.
RRS is not, as I have said, a cattle-killer. It will not rid the world of cows even if it does come to be found everywhere. It alone does not kill the Buffalo (perhaps you do not understand how fragile their survival was; they numbered in the hundreds at one point IOTL), and the idea that it originated among buffalo is just a theory in the TL. And its not correct.
The cobbling together of various prion disease elements to create RRS is of course, mere fancy on my part. I did construct the disease in a manner to fit the scenario I wanted. But I don't feel I stepped too far outside the realm of real-world prion diseases and research, and that others have jumped a bit too far in their own speculations.
As for human transmission, I am looking towards numbers broadly similar to that of BSE- a little bit higher but no more.