I think the most appropriate way of presenting these estimates is the HPD interval for the tMRCA conditioned independently on each of the rate estimates being the actual underlying rate. This is the approach we took in https://virological.org/t/1032/6
I still don’t think the BDBV rate is appropriate to use - the issue is that it is evolving in a different host species. Although the value is lower this is not necessarily because it is over a longer timespan, it could be because the rate in bats is different for all sorts of molecular and cellular reasons (or both).
As we also mention in the caveats to our post that we decline to report our estimated growth rates or derived metrics like R0 at the present because the HPDs include both zero and implausibly high rates. A mean or median of such an interval is potentially misleading. I believe it is better not to report it at this stage - if the HPDs aren’t meaningful then neither are the point estimates.