As for Biara and Railag, there are several opinions and possibilities.
First option.
Biara was sent to Igg-Shail to improve relations with the Warlocks and establish demon-worshipping sects there. To achieve this, she decided to gain the trust of the chieftain. And because there were several hints, and then he told her in her presence that he was Agrael, she recognized him. Personally, I think this option is unlikely.
Second option.
Biara, thanks to her intelligence, figured out how to simply rearrange the syllables in Railag's name. Then she decided to find an old acquaintance and use his trust to deliver Isabel to the Overlord and complete the mission. This is quite likely, as this succubus has intelligence and cunning.
I think this is the most likely scenario. There are many other options, but it would take too long to describe them all.