Query 1: Please ensure that you have configured measurement access for subscriber C on publisher A by selecting the appropriate option in the combo box on the Measurement Access screen.
Query 2: The procedure is pretty simple. Install the certificate into the certificate store, adjust permissions to give openHistorian access to the private key, use Windows tools to export the .cer file, then override the appropriate setting in openHistorian configuration to tell the system to use that .cer file to identify the local certificate. Note that GEP/STTP uses mutual authentication, so every system should have its own certificate whether subscriber or publisher. Please refer to the link in my previous post for information about the configuration settings that need to be overridden in order to use your own certificates. Here is the link again.
https://gridbits.gridprotectionalliance.org/t/subscriptions-using-tls/445