I doubt there’s been any such research. Who on Earth would fund studies into kefir as a candidiasis treatment in humans? There are a couple of scraps on PubMed to suggest anti-C. albicans properties .
Brown sugar promoted the greatest antimicrobial activities, producing inhibition halos corresponding to 35 … mm for Candida albicans … Different carbon source concentrations and the time of fermentation influenced the size of the inhibition halos of the pathogenic microorganisms.
Kefir and its insoluble polysaccharide, kefiran, were both tested for antimicrobial and cicatrizing activities against several bacterial species and Candida albicans … Both kefir and kefiran showed some activity against all organisms tested
If kefir is capable of supporting indigenous bacteria and lowering colon pH, then “die-off” is a potential possibility, even if it’s just endotoxin from other pathogens. There are a bunch of confounders though, like the lactose, casein and yeast that you mentioned, and the endotoxin and lactate from the kefir microbes themselves.
Kefir has also been shown to lower cholesterol, and unlike other infectious microbes, high cholesterol is something that increases candida’s pathogenicity. The fat soluble vitamins, minerals and components like lactoferrin that naturally occur in the milk, as well as the additional nutrients like vitamin K2 produced by the LAB, have their own health benefits. People on crazy candida diets don’t tend to consume any other source of dairy or nutritiously-dense food in general, so I don’t think it’s a good idea to encourage further exclusion unless absolutely necessary (dairy allergy, endotoxin sensitivity etc).