He has carried out many scientific experiments with it, but most of them haven't ended up in his book. You can find them here:
Research Archives | Price-Pottenger
If one wanted to prove that K2 is the Activator X he would have to repeat Weston Price's experiments with the same amount of K2 as found in his butter oil.
But do they match the population data? How many people in Weston Prices' studies consumed a high amount of butter? Only people in the Alps, as far as I remember. In the end, Weston Prices has chosen to focus on the diet of the Alpine people, but other people around the world were consuming many different diets, and they still all had healthy bones and teeth.
Yes, it makes sense why the Weston Price foundation would accept such a theory that would validate Price's work, even if it doesn't fully describe the phenomena.
Yes, his doses were much smaller that what people today have to use to see some benefits if they use just K2. That is one of the reasons why K2 doesn't fully fit in. Perhaps Activator X helps K2 as well?