1) "bind path" specifications from singularity.conf are effectively ignored, since htcondor passes "-C" to singularity.
I believe this is on purpose, since you may not want random strangers from the internet to have the same set of bind mounts as your local users.
2) SINGULARITY_BIND_EXPR does not really work as expected for me.
I'm pretty sure that the _EXPR means that the configuration value must be a valid ClassAd expression. For you, that probably means "a single ClassAd string". Try
SINGULARITY_BIND_EXPR = "/usr/libexec/condor/,/pool" instead. - ToddM