After 15+ years of this "ice 9" business I'm still waiting for results that in any way meet Koch's Postulates.
OK, this one will cover a lot of ground. Weber P et al. Cell-free formation of misfolded prion protein with authentic prion infectivity. Proc. Natl. Acad. Sci USA 2006 October 24; 103(43): 15818–15823. It's open-access, so no excuses about being stuck behind a paywall.
Making claims that biochemists working on prions "don't like to get dirty" is both insulting and disingenuous. Animal models are, in fact, used here to demonstrate that purified PrPsc (misfolded prion protein) is infectious in live hosts, in addition to triggering misfolding in vitro. No one uses farm animals because they're large, expensive, and there's no compelling reason to incur that cost when simpler model animals (here, hamsters) will do.
why not entertain the notion that this is a slow virus and that the symptomatic misfolded protein is a mere phenotype, possibly detrimental, but not causal
Well, because the linked paper was able to amplify the infective population of PrPsc in a cell-free system, which would not be conducive to the amplification of a virus.
I understand the appeal of an underdog hypothesis, but unless you can present a better argument that isn't comprised of ad hominems, vague conspiracy theories, and a smattering of scientific claims answered by 5-year-old literature, I'm not convinced.