Is the beta-glucan content an important criterion for comparing potential efficacy of commercial chaga supplements? Does the fact it was measurable in the product asked about also indicate that because beta-glucans were released during extraction, they will likely be bioavailable?

Both of these things are true and that is why this is used as a criterion in promotional material. The beta-glucan content reflects successful extraction of the active principal and its presence is thus a good predictor of potential benefit because the key to bioavailability is the release of beta-glucan and other ingredients from the mushroom through dissolving the chitin outer coating.